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Nd isotopic signatures and stratigraphic correlations : examples from western Pacific marginal basins… Mahoney, J. Brain 1994

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Nd ISOTOPIC SIGNATURES AN]) STRATIGRAPIIICCORRELATIONS: EXAMPLES FROM WESTERN PACIFICMARGINAL BASINS AND MIDDLE JURASSIC ROCKS OF THESOUTHERN CANADIAN CORDILLERAbyJ. Brian MahoneyB.S., University of Wisconsin-Madison, 1983M.S., Idaho State University, 1987A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTSFOR THE DEGREE OF DOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF GEOLOGICAL SCIENCESWe accept this thesis as conforming to the required standardTHE UNIVERSITY OF BRITISH COLUMBIAAugust, 1994© J. Brian Mahoney, 1994In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of Bntish Columbia, I agree that the Ubrary shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature______________________________Department of éZ)Lc2tcr <_tG’CLThe University of British ColumbiaVancouver, CanadaDate ‘‘ U ‘ld?,EYL ciDE-6 (2/88)ABSTRACTThe purpose of this investigation is twofold: 1) to evaluate the applicability of Nd and Sr isotopicanalyses of fine grained clastic sediments to basin analysis and stratigraphic correlation; and 2) to documentthe lithostratigraphic, biostratigraphic, geochemical and isotopic characteristics of Lower to Middle Jurassicstrata in tectonostratigraphic terranes of the southern Canadian Cordillera in order to evaluate potentialterrane linkages.Isotopic analyses of Neogene strata from three western Pacific marginal basins (Shikoku Basin, Sea ofJapan, Sulu Sea) permit evaluation of isotopic analyses to basin discrimination and stratigraphic correlation.The isotopic signatures of the Sulu Sea and Sea of Japan demonstrate that modern marginal basins have anisotopic signature that varies within limits defined by the geology of its source regions. The highly evolved(ENd < (-8)) isotopic signature of the Shikoku Basin, however, strongly overlaps that of the Sea of Japan, andcontrasts with the juvenile character of the crustal domains on the basin margins. This anomalous signature isinterpreted to be the result of cratonal aeolian influx. Temporal isotopic fluctuations in the Shikoku Basin areroughly synchronous across 5600 2 of basin floor, and the pattern of isotopic fluctuations can therefore beused to correlate strata throughout the basin. Isotopic fluctuations are interpreted to result from changes in therelative contribution of each crustal domain within the source region to the basins’ total sediment budget,which is a function of tectonism, volcanic episodicity, climatic factors, and basin hydrology. Isotopicfluctuations in a stratigraphic sequence may therefore prove to be important as both tools for stratigraphiccorrelation and as a monitor of basin evolution.Lithostratigraphic data indicate that Lower to Middle Jurassic strata of the Harrison, Cadwallader,Bridge River, and Methow terranes each contain six strikingly similar, regionally consistent lithostratigraphicvariations. Biostratigraphic data indicate that each terrane contains Aalenian to Bajocian strata with identicalmixed fauna of Boreal, East Pacific and Tethyan faunal realms. Isotopic data indicate that the Harrison,Cadwallader, and Methow terranes contain coeval isotopic fluctuations of similar magnitude. Volcanic11geochemical data indicate that the Harrison and Methow terranes constitute separate volcanic arc systemsflanking a basin containing back arc basin basalts. In addition, volcanic geochemistry and isotopic datasuggest that the Harrison terrane represents the youngest eastern fades of the eastward migrating Bonanza-Harrison arc system, which provides an Early Jurassic link between Wrangellia and Harrison terranes. Resultsof this investigation strongly suggest that Lower to Middle Jurassic strata of the Wrangellia, Harrison,Cadwallader, Bridge River, and Methow terranes comprise a single marginal basin floored by trapped oceaniccrust of the Bridge River terrane, and flanked by volcanic arc systems to the east and west. Wrangellia,Harrison, Cadwallader, Bridge River, and Methow terranes were amalgamated by the Early Jurassic, and havebehaved as a coherent crustal block since that time.II’TABLE OF CONTENTSABSTRACT iiTABLE OF CONTENTS ivLIST OF FIGURES viiLIST OF TABLES xLIST OF PLATES XiDEDICATION xiiACKNOWLEDGMENTS xiii1. INTRODUCTION 11.1 OBJECTIVE AND METHODS 6A. Lithostratigraphy 6B. Biostratigraphy 7C. Volcanic Geochemistiy 7D. Isotopic Characterization of Fine-Grained Sedimentaiy Rocks 7E. U-Pb Zircon Analyses 91.2 GOALS 91.3 PRESENTATION 102. ISOTOPIC FINGERPRINTING OF FINE-GRAINED CLASTIC SEDIMENTS:A CASE STUDY OF WESTERN PACIFIC MARGINAL BASINS2.1 iNTRODUCTION 132.2 BASiN DESCRIPTION 15A. SeaofJapan 17B. Shikoku Basin 19C. SuluSea 212.3 METHODS 22A. Sample Selection 22B. Age Control 24C. Analytical Procedures 24a. Rb-Sr 25b. Sm-Nd 25D. Data Presentation 262.4 RESULTS 272.5 DISCUSSION 35A. Interbasinal Isotopic Signatures 35B. Intrabasinal Isotopic Signatures 392.6 CONCLUSIONS 413. REGIONAL GEOLOGIC SEllING3.1 TERRANE DESCRIPTIONS 46A. Wrangellia 46B. Harrison 48C. Bridge River 49D. Cadwallader 51E.Methow 52ivF Cache Creek 54G. Quesnellia 563.2 STRUCTURAL SET1TNG 573.3 PLUTONIC SETrING 604. EVOLUTION OF A MIDDLE JURASSIC VOLCANIC ARC: STRATIGRAPHIC, ISOTOPICAND GEOCHEMICAL CHARACTERISTICS OF THE HARRISON LAKE FORMATION,SOUTHWESTERN BRITISH COLUMBIA4.1 INTRODUCTION 644.2 GEOLOGIC SETTiNG 644.3 PREVIOUS WORK 674.4 STRATIGRAPHY 67A. Celia Cove Member 68B. Francis Lake Member 70C. Weaver Lake Member 72D. Echo Island Member 754.5 AGE CONSTRAINTS 77A. Biostratigraphic Data 77B. U-Pb Geochronology 784.6 STRUCTURAL DEFORMATION 824.7 GEOCHEMISTRY 834.8 Nd-Sr ISOTOPIC SYSTEMATICS 934.9 ALTERATION 984.10 MINERALIZATION 984.11 EVOLUTION OF HARRISON LAKE FORMATION 994.12 MODEL FOR VOLCANIC ARC DEVELOPMENT 1024.13 CONCLUSIONS 1035. EARLY TO MIDDLE JURASSIC VOLCANISM ON WRANGELLIA:EVOLUTION OF THE BONANZA-HARRISON ARC SYSTEM5.1 INTRODUCTION 1075.2 GEOLOGIC SETTING 1095.3 TERRANE STRATIGRAPHY 1115.4 LOWER TO MIDDLE JURASSIC STRATA 1145.5 AGE CONSTRAiNTS 1165.6 GEOCHEMISTRY 117A. Major and Trace Element Geochemistiy 124B. Rare Earth Elements 1315.7 ISOTOPIC SIGNATURE 1365.8 CONSTRAINTS ON ARC CORRELATION 141A. Lithostratigraphic Considerations 141B. Temporal Considerations 143C. Geochemical Considerations 144D. Isotopic Considerations 145E. Structural Considerations 1455.9 CONCLUSIONS 1476. REGIONAL TECTONOSTRATIGRAPHIC CORRELATIONS IN THESOUTHERN CANADIAN CORDILLERA: IMPLICATIONS FOR JURASSICTERRANE LINKAGES AND BASIN EVOLUTION6.1 INTRODUCTION 1496.2 GEOLOGIC SETTING 151A. Terrane Distribution 1526.3 STRATIGRAPHIC CHARACTERIZATION 156A. Harrison Terrane 156Va. Terrane Description 156b. Lithostratigraphy 157c. Biostratigraphy 159d. Volcanic Geochemistry 160e. Isotopic signature 160f Depositional Environment 165B. Cadwallader Terrane 166a. Terrane Description 166b. Lithostratigraphy 167c. Biostratigraphy 170d. Volcanic Geochemistry 171e. Isotopic signature 172J Depositional Environment 172C. Bridge River Terrane 175a Terrane Description 175b. Lithostratigraphy 177c. Biostratigraphy 181d. Volcanic Geochemistry 183e. Isotopic signature 186f Depositional Environment 189D. Methow Terrane 191a. Terrane Description 191b. Lithostratigraphy 1921. Boston Bar Formation 1922. Dewdney Creek Formation 195c. Biostratigraphy 198d. Volcanic Geochemistry 199e. Isotopic signature 201f Depositional Environment 2036.4 STRATIGRAPHIC CORRELATIONS 207A. Lithostratigraphic Correlations 207B. Biostratigraphic Correlations 212C. Volcanic Geochemistiy Correlations 213D. Isotopic Signatures 2166.5 BASIN EVOLUTION MODEL 2207. CONCLUSIONS 230a. Tectonic Implications 2348. REFERENCES 238APPENDICES 254APPENDIX A - ANALYTICAL TECHNIQUES 254APPENDIX B - ANALYTICAL PRECISION 258APPENDIX C - SAMPLE LOCATIONS 264APPENDIX D - TIHN SECTION DESCRIPTIONS 277APPENDIX E - CURRENT RESEARCH PUBLICATIONS 291El. Mahoney, 1991 292E2. Mahoney, 1992 298E3. Mahoney and Journeay, 1993 308E4. Journeay and Mahoney, 1994 318viLIST OF FIGURESFigure 1.1 - (a) - Morphogeologic map of the Canadian Cordilera; (1,)- Schematicterrane map of the Canadian Cordillera 2Figure 1.2 - Schematic terrane map of the southern Canadian Cordillera 4Figure 2.1 - Geographic map of western Pacilic marginal basins, with crustal isotopic domains 16Figure 2.2 - Schematic stratigraphic sections of Neogene sediments inwestern Pacific marginal basins 18Figure 2.3- Schematic stratigraphic sections of Neogene Shilcoku Basin sediments 20Figure 2.4- Nd VS. 87SrI6r isotopic diagram for three western Pacific marginal basins 28Figure 2.5 - Depth vs. ENd for drill holes in the Shikoku Basin 32Figure 2.6- Age vs. ENd for drill holes in the Shikoku Basin 33Figure 2.7 - Age vs 87Sr/6rfor drill holes in the Shikoku Basin 34Figure 2.8 ..jSm/Nd vs. ENd for samples from the Shikoku Basin and Sea of Japan 33Figure 3.1 - Geographic location map of the southwestern British Columbia 45Figure 3.2 - Generalized geologic map of southwestern British Columbia 47Figure 4.1 -Schematic terrane map of the southern Canadian Cordillera and generalizedstratigraphic column of the Harrison terrane 65Figure 4.2 - Geologic map of the southwestern 1/4 of Harrison Lake (92H/5)1:50,000 map sheet (in pocket)Figure 4.3 - Schematic stratigraphic column of the Harrison Lake Formation 69Figure 4.4- Photomicrograph of altered crystal tuff of Francis Lake Member 74Figure 4.5- Photograph of columnar jointed dyke cutting altered lava flow inWeaver Lake Member 74Figure 4.6- Concordia plots of U-Pb dates discussed in text 79Figure 4.7 - Major and trace element discrimination diagrams ofWeaver Lake Member volcanic rocks 86Figure 4.8 - Harker variation diagrams for the Weaver Lake Member 88Figure 4.9- Compatible-incompatible element diagrams for the Weaver Lake Member 89Figure 4.10- Ta-Hf/3-Th discriminate diagram for Weaver Lake Member 91viiFigure 4.11 - Incompatible element spider diagrams and REE diagrams for theWeaver Lake Member 92Figure 4.12 - 87SrI86Sr vs. ENd diagram for Harrison Lake Formation 94Figure 4.13 - estimated stratigraphic vs. ENd position for Harrison Lake Formation 97Figure 5.1 - Schematic terrane map of the southern Canadian Cordillera, highlightingWrangellia and Harrison terranes 108Figure 5.2 - Schematic geologic map of Bonanza Group, Bowen Island Group,and Harrison Lake Formation 110Figure 5.3 - Time-stratigraphic column of the Bonanza Group, Bowen Island Group,and Harrison Lake Formation 112Figure 5.4- AFM and ZrITiO2vs. Si02 for Bonanza Group, Bowen Island Group,and Harrison Lake Formation 125Figure 5.5 - Harker variation diagrams for Bonanza Group, Bowen Island Group,and Harrison Lake Formation 126Figure 5.6- Ti02 vs. Al203and Ti02 vs. Si02 diagrams for Bonanza Group, Bowen IslandGroup, and Harrison Lake Formation 128Figure 5.7 - Incompatible element ratio diagrams for Bonanza Group, Bowen IslandGroup, and Harrison Lake Formation 130Figure 5.8 - Rare earth element diagrams for Bonanza Group, Bowen Island Group,and Harrison Lake Formation 132Figure 5.9 - Comparison of REE subgroups for each unit 134Figure 5.10 - 87Sr/6rvs. ENd diagram for Bonanza Group, Bowen Island Group,and Harrison Lake Formation 137Figure 5.11- ENd VS. jSml’ diagram for Bonanza Group, Bowen Island Group,and Harrison Lake Formation 140Figure 6.1 - Schematic terrane map of the southern Canadian Cordillera 150Figure 6.2 - Schematic geologic map of Lower to Middle Jurassic strata in thesouthern Canadian Cordillera. Modified from Wheeler and McFeely (1991) 153Figure 6.3 - Schematic geologic cross-section across southern Canadian Cordillera.Modified from Monger and Journeay (1992) 154Figure 6.4. - Time-stratigraphic column of terranes in the southern Canadian Cordillera 155Figure 6.5- Schematic stratigraphic section of the Harrison Lake Formation 158ViiiFigure 6.6 - Representative geochemical diagrams for the Weaver Lake Memberof the Harrison Lake Formation 161Figure 6.7 - 87SrI6r vs. ENd diagram for Harrison Lake Formation 163Figure 6.8 - estimated stratigraphic position vs. ENd for Harrison Lake Formation 164Figure 6.9 - Schematic stratigraphic section of the Last Creek Formation 168Figure 6.10 - 87Sr/6rvs. ENd isotopic diagram for Last Creek Fonnation, 173Figure 6.11 - stratigraphic position vs. ENd for Last Creek Formation 174Figure 6.12 - Schematic stratigraphic section of the Cayoosh Assemblage 178Figure 6.13 - Photograph of pillow basalt from unit 2 of Cayoosh assemblage 180Figure 6.14 - Photograph of thin-bedded quartz-rich clastic facies displayingchaotic bedding character 180Figure 6.15 - Representative geochemical diagrams for volcanic rocks of unit 2of the Cayoosh assemblage 184Figure 6.16- 87Sr/6rvs. ENd isotopic diagram for Cayoosh assemblage 187Figure 6.17 - stratigraphic position vs. ENdfor Cayoosh assemblage. 188Figure 6.18 - Schematic stratigraphic section of the Ladner Group 193Figure 6.19 - Representative geochemical diagrams for volcanic rocks ofDewdney Creek Formation 200Figure 6.20- 87Sr/6rvs. ENd isotopic diagram for Dewdney Creek Formation 202Figure 6.21- stratigraphic position vs. Nd for Dewdney Creek Formation. 204Figure 6.22- Lithostratigraphic correlation diagram for Lower to Middle Jurassic strata 210Figure 6.23- H173-Th-Ta tectonic discrimination diagram for Middle Jurassic volcanic strata 215Figure 6.24- ENd jSm1Nd diagram for all Lower to Middle Jurassic strata examined 219Figure 6.25 - Schematic Lower to Middle Jurassic basin reconstruction 222ixLIST OF TABLESTable 1.1 - Terrane description table 5Table 2. la- Isotopic data for western Pacific marginal basins 29Table 2. lb - Isotopic data for western Pacific marginal basins 30Table 4.1 - U-Pb analytical data for Harrison Lake Formation 80Table 4.2a - Geochemical data for the Harrison Lake Formation 84Table 4.2b - Geochemical data for the Harrison Lake Formation 85Table 4.3 - Isotopic data for the Harrison Lake Formation 95Table 5. Ia - Geochemical data for the Bonanza Group in the Port Alberm area 118Table 5. lb -Geochemical data for the Bonanza Group in the Nootka Sound area 119Table 5. lc - Geochemical data for the Harrison Lake Formation 121Table 5. Id - Geochemical data for the Bowen Island Group and Middle Jurassic plutons 123Table 5.2 - Isotopic data for the Bonanza Group, Bowen Island Group,and the Harrison Lake Formation 138Table 6.1 - Paleontologic data for the southern Canadian Cordillera 224Table 6.2 - Middle Jurassic volcanic geochemical data 226Table 6.3 - Lower to Middle Jurassic isotopic analysis data table 227Table B. 1 - Duplicate Analyses for Ocean Drilling Program sediments 259Table B.2- Duplicate Analyses for Jurassic rocks 261Table 8.3 - International Standards 263xLIST OF PLATESPlate 1 - Geologic map of the southwestern Canadian Cordillera (92G, H, I, J) with Nd and Sr isotopicsample locations and values. Scale 1:500,000. Geology compiled by J.M. Journeay and J.W.H.Monger; isotopic data compiled by J.B. Mahoney.xiDEDICATIONI would first and foremost like to dedicate this thesis to Lori D. Snyder, the special womanin my life, without whom this thesis would not have been possible. I can never thank her enough forher help and encouragement through good times and bad. Thanks, Lori.A special dedication to my sister, Chris, who was unable to see me finally graduate fromcollege. Chris is sorely missed by all. Only the good die young.XIIACKNOWLEDGMENTSI would first like to thank the late RL. Armstrong for introducing me to the geology of theCanadian Cordillera and for pointing out the problems and possibilities associated with Jurassicstratigraphy in the region. Dick and I discussed the potential application of radiogenic isotopes tostratigraphic studies endlessly as I was beginning this project, and the research herein is a test of thoseideas. My thanks to Marc Bustin for becoming my thesis advisor following Dick’s passing, and to JohnRoss for his interest in the project. Bill Barnes is especially thanked for helping to keep it all inperspective over the last few months.This work would not have been possible without the generous support, encouragement, andscientific contributions of the Geological Survey of Canada. Cathie Hickson provided logistical supportand field expenses, and was continually my sounding board for a vast array of ideas and problems,scientific and otherwise. My sincerest thanks are extended to Murray Journeay for his motivation,enthusiasm and expertise in Coast Belt geology; many of the ideas discussed in this thesis were developedwhile working with Murray in the eastern Coast Belt. Murray is also thanked for the use of his excellentdigital map files that have been modified and used herein. I would like to thank Jim Monger for askingthe thought-provoking questions about regional geology and the feasibility of my interpretations.Rich Friedman kept me grounded in geologic reality over the past couple of years, and served asa filter for every wild idea that came up; I cannot thank him enough for his interest, enthusiasm andassistance. Jim Mortensen timed his arrival at UBC to direct the isotope lab perfectly, and has been aconstant source of ideas and necessary critiques. His assistance is greatly appreciated.This project involved an enormous amount of laboratory work, and I owe a debt to numerouspeople for their assistance. My thanks to Dita Runkle for helping with a myriad of lab activities, and toxliiJanet Gabites for keeping me from blowing up the mass spectrometer. Anne Pickering kept things livelyand entertaining in the lab, and always kept a smile on my face.Neodymium analyses were done at the isotopic lab of the Geological Survey of Canada, Ottawa.I would like to thank Reg Theriault, neodymium guru, for his help, advice, and isotopic expertise. Mythanks to Randy Parrish for authorizing my use of the lab, and for just being a friend.Sophie Weldon provided invaluable assistance in helping me organize megabytes of data. LoriSnyder was a constant source of computer expertise, geochemical assistance, and manuscript editing.Arnie Toma helped out with computer drafting when time was short. Finally, my thanks to other facultyand staff in the Department of Geological Sciences who provided the expertise needed to produce thistome.xiv1. INTRODUCTIONThe Canadian Cordillera is a collage of fault-bounded terranes which have undergone varyingdegrees of deformation, metamorphism, and plutomsm during Jurassic to Tertiary time (Wheeler andMcFeely, 1991). Each terrane is defined on the basis of a characteristic internal stratigraphy that differs fromthat of its neighbor, in ways that are not easily explained by conventional facies changes or unconformablerelations (Coney et al., 1980). The original temporal and spatial relationships among terranes, initialpaleogeographic affinity, transport history and timing of accretion are poorly understood. Current theoryholds that the Cordillera developed through the progressive accretion of numerous, unrelated, allochthonousterranes onto the western margin of ancestral North America during the Mesozoic (Coney et al., 1980; Mongeret al., 1982, Silberling and Jones, 1987; Monger, 1989). These accreted terranes were subsequently variablydeformed, intruded, and metamorphosed during a long and protracted geologic history that formed thecomplex orogen recognized today.Current debate on the geology of the Canadian Cordillera centres on the tectonic setting,displacement history, and timing of accretion of the allochthonous terranes that comprise the Cordillera (Figs.1. Ia, b). Tectonic models for accretion fall into three categories: 1) late Early Cretaceous collision of InsularSuperterrane (amalgamated Wrangellia, Alexander and Peninsular terranes or Terrane II of Monger, et al.,1982) with previously accreted terranes along the western edge of North America and closure of interveningocean basins (Monger et al., 1982; Garver, 1992; Brandon and Garver, 1994); 2) Late Jurassic dextraltranslation and oblique transpression of Insular Superterrane along the North American margin withconcomitant development of transtensional rift basins (Gehrels and Saleeby, 1985; Saleeby and Busby-Spera,1992); and 3) Middle Jurassic accretion of Insular Superterrane and formation of an Andean-Sierran typecontinental arc, followed by Late Jurassic to late Early Cretaceous intra-arc rifling and basin formation (vander Heyden, 1992). Each of these models attempts to integrate substantial igneous, metamorphic, structural,and stratigraphic data sets into a comprehensive framework.1CRATONENorihArn.riconICR RAN C SKOSm *rcmC*,i SAStIiCN??Monosh..O5PLActO COA?IWLNTh.E:iCossarLIN(lIingP(*ICSsTQ.SCKoetenayACCRCTCO TERRANESwnCIiowue 5UP(*TCIIAACSlid. haoun*omOors.yjEJOu.sneia1:.JCoeh. CreikJStIkInioTCflACS Cl TUC COAST kL?CodwoUad.rRiv.rIWS&5.AI S4ØCPTCAIAN(JAI.zond.OU?tR TER*AIICS1ChugochYakutotBATHOUTHSCoo5t PlulonicComprnsFigure 1.1(a)- Morphogeologic map of the Canadian Cordillera. Modified after Monger and Hutchinson(1971); (b) Schematic terrane map of the Canadian Cordillera. Modified after Wheeler and McFeely(1991).(b)2Fundamental to any tectonic model of Cordilleran evolution is the establishment of linkages betweenterranes. Documentation of terrane linkages dates the earliest possible juxtaposition of disparate terranes, andthus provides constraints on the timing and mechanism of terrane accretion. Terrane linkages can beestablished by the documentation of stratigraphic overlap assemblages, identification of “stitching” igneousintrusions or metamorphic isograds, and provenance ties. Comprehensive tectonic models for the southernCanadian Cordillera have been contentious due to disagreements among workers regarding linkages amongterranes (e.g. Umhoefer, 1989; Garver, 1992; Monger and Journeay, 1992).The southern Canadian Cordillera provides an ideal setting within which to test the feasibility ofcurrent tectonic models. This portion of the Cordillera consists of a number of small, disparate terranesseparated by regional fault systems or plutonic assemblages (Figs. 1.1 and 1.2, Plate I). These terranesinclude, from west to east, Wrangellia, Harrison, Cadwallader, Bridge River, Methow, and Quesnellia terranes(Table 1.1 Fig. 1.2). Each of the terranes contains Jurassic-Cretaceous basinal sequences overlying Triassic orolder basement (Monger and Journeay, 1992). Stratigraphic linkages among these terranes are firmlyestablished by the late Early Cretaceous (Albian; Garver, 1992), but proposed correlations of older strata are amatter of dispute (Mahoney, 1993; Mahoney and Journeay, 1993; Journeay and Mahoney, 1994, Garver andBrandon, 1994). Documentation of earlier terrane linkages would better constrain currently proposed tectonicmodels.This investigation attempts to reconcile conflicting tectonic models by establishing the stratigraphicevolution and basin configuration of Middle Jurassic strata in a number of terranes in the southern CanadianCordillera. These strata (and their terrane designations) include, from east to west, the Ashcroft Formation(Quesnellia), Ladner Group (Methow terrane), Lillooet Group (Methow terrane), Newby Group (Methowterrane), Cayoosh assemblage (Bridge River terrane), Last Creek Fonuation/Relay Mountain Group(Cadwallader terrane), and Harrison Lake Formation (Harrison terrane). Terrane descriptions are outlined inpoint form in Table 1.1. Documentation of Middle Jurassic stratigraphic ties between these formations wouldestablish the earliest terrane linkages known in the southern Cordillera, and would therefore place important3Figure 1.2 - Schematic terrane map of the southern Canadian Cordillera. Modified after Wheeler andMcFeely (1991).AAAAlAAAAA/ /7 / / // / // // / / / / // // / / // / // // / / // / /// / / // / /•/ / /‘/ / // / / // / // / // / /WrangelliaHarrisonCadwalladerBridge RiverMethowQuesnelliaCache CreekStikiniaKootenayCassiarterranes of Northwest CascadesCoast Flutonic Complexundivided metasedimentsof the eastern Coast BeltOmineca Crystalline ComplexI— /IAAAAAAAAAAAAAAAAAA___BCAAAAAAAAAAAAAAAAAAAAAA A A A A A A A‘‘ASHA AA A AA AAA AAA A A,\‘ ..AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\‘‘ AAAAAAAAAAAAAAAAAAA4TABLE 1.1TERRANE NOMENCLATUREThis investigation concentrates on the stratigraphic evolution and basin histories of Middile Jurassicstrata contained in a number of disparate terranes in the southern Canadian Cordillera. These terranesinclude, from east to west, Quesnellia, Methow, Bridge River, Cadwallader, Harrison, and Wrangelliaterranes. They are described in point form below; refer to figure 1.2 for terrane locations.Quesnellia:Fragmented record of Upper Paleozoic carbonate platform (?) and ocean floor rocks (ApexMountain Group and Harper Ranch Group); overlain by Upper Triassic Nicola Group volcanic arc;unconformably overlain by Lower to Middle Jurassic fine-grained clastics of Ashcroft Formation; overlain bylate Early Cretaceous Spences Bridge Group continental arc volcarncs; capped by Eocene (Kamloops Group)and Miocene/Pleistocene (Chilcotin Group) plateau basalts. Intruded by Triassic/Early Jurassic throughEocene plutons (Monger, 1989, 1990).Methow:Upper Triassic ocean floor chert, limestone, greenstone (Spider Peak Formation; (Ray,1986)) overlain by Lower Jurassic Boston Bar Formation fine-grained clastics, which grade upward intovolcaniclastic rocks of the Dewdney Creek Formation (O’Brien, 1986; Mahoney, 1992); overlain by fluvialrocks of Upper Jurassic Thunder Lake assemblage; overlain by coarse clastics of the Lower to mid-CretaceousJackass Mountain Group; intruded by Cretaceous to Eocene plutons (Monger, 1989).Bridge River:Disrupted, variably metamorphosed argillite, greenstone, ultramafics, carbonate, and chertranging in age from Mississippian to Middle Jurassic (Callovian; Cordey and Schriazza, 1993); overlain byCretaceous clastic rocks of the Tyaughton Group (Schriazza et al., 1991; Journeay, 1993); intruded byCretaceous and Eocene plutons.Cadwallader:Upper Triassic basaltic rocks of the Cadwallader Group (Rusmore, 1985, 1987) overlain byLower Jurassic fine-grained clastics of the Last Creek Formation, Upper Jurassic volcanogenic clastics of theRelay Mountain Group (lJmhoefer, 1990), and Lower to Upper Cretaceous coarse clastic rocks of the TaylorCreek Group and Silverquick Formation (Garver, 1992); capped by continental arc volcarncs of the UpperCretaceous Powell Creek Formation; intruded by Cretaceous to Eocene plutons (Journeay, 1993).Harrison:Upper Triassic greenstone, greywacke and chert (Camp Cove Formation; Arthur, 1993)unconformably overlain by Middle Jurassic volcanic rocks of the Harrison Lake Formation, fine-grainedsedimentaiy rocks of the late Middle Jurassic Mysterious Creek and Billhook Creek Formations, LowerCretaceous coarse clastic rocks of the Peninsula Formation, and Lower Cretaceous volcanic rocks of theBrokenback Hill Formation (Arthur, 1986, 1993); intruded by Middle Jurassic to Cretaceous plutons.Wrangellia:Devonian Sicker Group volcanic arc overlain by Upper Paleozoic carbonate platform;Paleozoic rocks unconformably overlain by Middle to Upper Triassic Karmutsen Formation tholeiitic basalt(oceanic plateau), in turn overlain by Lower Jurassic Bonanza Group volcanic arc and Upper Jurassic to UpperCretaceous clastic sediments (Monger, 1980); intruded by Middle Jurassic to Eocene plutons.5constraints on models of tectonic evolution. Conversely, documentation of substantially different stratigraphichistories among these formations would preclude amalgamation of these terranes prior to the Late Jurassic.1.1 OBJECTIVE AND METHODSThe primary objective of this investigation is to constrain the timing and sequence of terraneaccretion in the southern Canadian Cordillera through stratigraphic correlations and facies analysis of Middleto Upper Jurassic volcanic arc and marginal basin assemblages. Jurassic strata in the region are contained in anumber of terrane specific, unconformity-bounded sedimentaiy sequences separated by regional fault systemsand plutonic assemblages. Original relationships between these sequences are uncertain. The basinalassemblages should contain evidence of terrane interaction in the form of unconformities, change inprovenance, vertical facies variations, and the development of volcanogenic sequences as a result of thetectonic instability inherent in an accretionaiy tectonic setting. Documentation of geologically simultaneouslithologic, biostratigraphic, isotopic and geochemical changes in coeval sedimentary sequences that arecurrently tectonically separated would provide strong evidence for their original linkage. Evaluation of Middleto Late Jurassic basin configuration and evolution will therefore place constraints on terrane interactions andsubsequent tectonic models for this time period.Jurassic strata are examined and compared by a variety of stratigraphic techniques, includinglithostratigraphy, biostratigraphy, isotopic characterization, geochemical analysis of primary volcanic rocks,and, to a lesser extent, U-Pb zircon analyses. Each of these techniques is described below:A. Lithostratigraphy -Physical stratigraphy and sedimentology fonns an important component of this investigation. Most ofthe units under consideration have been examined in only a cursoiy manner by previous workers, and detailedstratigraphic descriptions are lacking. Multiple stratigraphic sections were examined and sampled in eachunit (Appendix E), and lateral and vertical facies changes are documented throughout the area of exposure.6Thin section petrography was used to document compositional variations (Appendix D). Particular emphasisis placed on depositional environment interpretation and provenance determination for each unit.B. Biostratigraphy -Paleontologic data are used to refine the age range of each unit. New fossil dates are reported in theLadner Group, Lillooet Group, Cayoosh assemblage, and the Harrison Lake Formation. These data add toexisting biostratigraphy, and, in most cases, increase the age range of the unit concerned. Biostratigraphy alsoaids in ecological interpretation, and serves to limit the paleogeographic separation of the terranes.C. Volcanic Geochemistry -Major, minor, and trace element geochemistry of primary volcanic rocks, primarily lava flows, hasbeen undertaken in an attempt to discriminate between coeval volcanic arc assemblages. The Harrison LakeFormation and the Ladner Group both contain thick volcanic sequences, and the original paleogeographicrelations between these arc-related assemblages is unknown. Differences in volcanic arc geochemistry may berelated to differences in source areas, particularly differences in assimilant contamination. Differentgeochemical signatures, in concert with isotopic data, suggest different basement compositions, and thereforedifferent terrane affiliations. Conversely, similar geochemical signatures between arc assemblages suggestsimilar source areas, potentially within the same arc.D. Isotopic characterization of fine-grained sedimentary rocks -Delineation of isotopic signatures of sedimentary sequences is an integral part of this investigation.Radiogenic isotopic (Nd and Sr) data from fine-grained sedimentary sequences may aid in stratigraphiccorrelation and provenance determinations in Middle Jurassic strata. The viability of such isotopiccorrelations has been investigated in a pilot study of Neogene sediments in western Pacific marginal basins(Chapter 2). An introduction to the theory behind isotopic characterization of sedimentary strata is presentedbelow.7Fine-grained sedimentary strata provide a unique challenge in stratigraphic studies due to themonotonous, non-distinctive character of the rocks, the paucity of sedimentary structures, and the absence ofunique provenance indicators. This problem is compounded in geologically complex accreted terranes, wherethe original paleogeographic relationships among coeval strata are obscured by major crustal structures andigneous complexes. Radiogenic isotopic analysis of fine-grained sedimentary strata should provide a distinctisotopic signature that may be utilized as a stratigraphic correlation tool among coeval strata in geologicallycomplex areas where traditional stratigraphic methods are of dubious value. A portion of this investigationexamines the feasibility of using radiogenic isotopes (Sm/Nd, Sr) to “fingerprint” fine-grained sedimentaryrocks for use as a tool in stratigraphic correlation, and therefore as an aid in basin analysis and tectonicreconstructions.The applicability of radiogemc isotopic correlation of fine-grained sediments to unraveling thestratigraphic and tectonic history of the southern Canadian Cordillera is unknown, and requires a test study.Neogene sediments from modern marginal basins provide a well-constrained sample group with which to testthe validity of radiogenic isotope characterization. Drill core retrieved by the Ocean Drilling Programprovides well-dated, precisely located sediment samples from modern marginal basins in known tectonicsettings with known provenance. Fine-grained Plio-Pleistocene sediments have been retrieved from threedifferent basins (Sea of Japan, Philippine Basin, Sulu Sea). Whole-rock isotopic analyses provide dataregarding the lateral and vertical variations in isotopic signatures within these sediments. Fine-grainedsediments of limited stratigraphic range in any one basin should display approximately similar radiogemcisotope signatures, reflecting provenance, and this signature should differ from the signal of coeval sedimentsin other basins, due to differences in provenance crustal history. Data collected from this study of modernmarginal basin sediments will determine the viability of isotopic correlations in older, less well-constrainedstrata of the southern Canadian Cordillera. The results of this pilot study are described in Chapter 2.SE. U-Pb zircon analyses -Provenance studies are particularly important in the identification of any detritus of continentalaffinity (i.e. older) within the basinal assemblages. Isotopic signatures (Nd and Sr) will identil’ any extremelyold (i.e. Precambrian) material., but the identification of Late Paleozoic or Mesozoic detritus requires zirconprovenance studies. Both the Cayoosh assemblage and the Ladner Group contain sections of quartzofeldspathic sandstone of unknown affinity. Detrital zircon studies may provide definitive provenanceinformation. In addition, primary emplacement ages on volcanic sequences in the Harrison Lake Formationare determined by U-Pb (zircon) geochronology.Taken alone, each of these techniques can provide only circumstantial evidence of stratigraphiccorrelation. However, utilization of a variety of techniques provides multiple lines of evidence that may beused in support (or refutation) of proposed stratigraphic correlations.1.2 GOALSThis investigation specifically addresses the following questions:1. Are radiogenic isotopes (Sr and Nd) useful in stratigraphic characterization and correlation ofbasinal sequences, and can they be used in paleogeographic reconstructions of allochthonous terranes?2. What is the character of each Middle Jurassic stratigraphic sequence, and how does thestratigraphic evolution in each sequence compare to that of its neighbors?3. Were these Middle Jurassic strata originally deposited in different basins that were sequentiallyadded to the continental margin, or were these strata originally parts of a single Middle Jurassic basin that hasbeen subsequently dismembered?94. What is the earliest age at which continental detrital input, and therefore continental proximity,can be proven?1.3 PRESENTATIONThis thesis is divided into four interrelated segments, and the results of each are presented as anindividual manuscript, in accordance with university guidelines. Preparation of each segment as an individualmanuscript facilitates subsequent publication, but results in unavoidable overlap and repetition.The first segment consists of a pilot study into the viability of radiogenic isotopic correlation of finegrained sediments in modern marginal basins of the Western Pacific. This pilot study is undertaken toevaluate the applicability of Nd and Sr isotopic analyses to stratigraphic correlation and basin discriminationin a well constrained basinal setting. Successful application of isotopic correlations in modern basins willsupport the use of the technique in Jurassic strata of the southern Canadian Cordillera. The remainder of thethesis documents the geologic setting of terranes in the southern Canadian Cordillera and proposescorrelations among them.The second thesis section characterizes the geology and geochemistry of the Harrison terrane, whichforms the eastern edge of the westernmost (modern geography) volcanic arc in Middle Jurassic time. Thegeology, geochemistry, and isotopic signature of the Middle Jurassic Harrison Lake Formation has not beenpreviously documented in detail. These characteristics will be used for comparison with adjacent basinalsequences.The third section attempts to link the Harrison terrane with slightly older arc assemblages onWrangellia to the west. The geology, geochemistry, and isotopic signature of Middle Jurassic volcanic stratain Wrangellia is examined in detail and compared to Middle Jurassic volcanic strata in the Harrison terrane toevaluate potential correlations between the volcanic units. Links between Wrangellia and terranes to the east10have not been documented prior to this investigation. This link is important for documenting the originalaerial extent of Wrangellia and the heterogeneity of its basement rocks.The final, and most extensive, portion of this investigation describes Middle Jurassic stratigraphicevolution and proposes terrane linkages among Harrison, Cadwallader, Bridge River and Methow terranes,and develops a model of Middle Jurassic basin development in the southern Canadian Cordillera. Thelithostratigraphy, biostratigraphy, volcanic geochemistry and isotopic signature of Lower to Middle Jurassicstrata on each terrane are examined in detail, and correlations are proposed on the basis of similaritiesdocumented between the terranes. This investigation represents the first systematic regional examination ofLower to Middle Jurassic strata on terranes of the southern Canadian Cordillera.11CHAPTER 2ISOTOPIC FINGERPRINTING OF FINE-GRAIKED CLASTIC SEDIMENTS:A CASE STUDY OF WESTERN PACIFIC MARGINAL BASINS122. ISOTOPIC FINGERPRINTING OF FINE-GRAINED CLASTIC SEDIMENTS: A CASE STUDY OFWESTERN PACIFIC MARGINAL BASINS2.1 INTRODUCTIONFine-grained sedimentary strata provide a unique challenge in stratigraphic studies due to theirtypically monotonous, non-distinctive character, lack of sedimentary structures, and absence of distinctiveprovenance indicators. Traditional sedimentologic techniques (stratigraphy, petrography) rely on coarsergrained materials, which bias the data toward local sediment sources, and ignore more distal sources ofsediment that may be of equal or greater importance in basin evolution and paleogeographic reconstructions.Detailed analyses of flne-grained strata, which form the bulk (>60%) of the global sedimentary mass, shouldyield important data relevant to stratigraphic correlation and the evolutionary history of individual basins.Fine-grained deep marine sediments are the product of pelagic and hemipelagic deposition within asedimentary basin and comprise sediment primarily transported to the basin in suspension by fluvial or aeolianprocesses. These processes tend to uniformly sample a given source region, and the long transportationhistories of fine-grained materials lead to a complete mixing of heterogeneous detrital components, producingsediment with a composition that approximates the average crustal composition of a lithologicallyheterogeneous source region (Taylor and McLennan, 1985). The original average crustal composition of thesource region may be modified, however, as most major and minor elements undergo some degree ofgeochemical modification during the sedimentary cycle (Taylor and McLennan, 1985; McLennan and Taylor,1991). Numerous studies have demonstrated that the rare earth element (REE) composition of flne-grainedsedimentary rocks closely mirrors that of the source region, indicating that little or no REE fractionationoccurs during the sedimentary cycle (Nance and Taylor, 1976; Goldstein et al., 1984; Taylor and McLennan,1985; Miller et al., 1986; Andre et al., 1986; Grousset et al., 1988). Geochemical and isotopic variations in theREE composition of fine-grained strata through time therefore record temporal variations in the sediment fluxwithin any given basin, and may be useful stratigraphic markers for deciphering basin evolution.13Neodymium isotopic analyses of fine-grained sediments have been successfully utilized in bothprovenance determination and modeling crustal growth and evolution (Allegre and Rousseau, 1984; Goldsteinet al., 1984; Miller and O’Nions, 1984; Michard et al., 1985; Miller et al., 1986; Goldstein and Jacobsen,1988, McLennan et al., 1990). The majority of previous studies have focused on Nd isotopes as geochemicalindicators of crustal evolution or on long tenn isotopic variations applicable to tectonic reconstructions (Millerand O’Nions, 1984; Andre et a!, 1986; Frost and Coombs, 1989). The isotopic character of sediments from avariety of tectonic settings was examined by McLennan et al. (1990) to test the relationship between tectonicsetting and provenance. The use of variability of the Nd-Sr isotopic signatures within basinal strata as a toolin stratigraphic studies and basin evolution and reconstruction has never been rigorously tested.The isotopic composition of fine-grained sediments should be controlled by the relative contributionof material from the various source terranes feeding the depositional basin. Temporal variation in isotopiccomposition in a stratigraphic sequence may record the response of a depositional basin to evolution of thesource terranes. If variations in the isotopic signature of fine-grained strata record the evolution of the sourceterranes, analysis of this signature can potentially provide invaluable information regarding basin developmentand evolution.The Nd isotopic signature of modern deep sea sediments fluctuates within relatively restricted limits,reflecting the domination of continentally derived fluvial influx (Goldstein and Jacobsen, 1987, 1988).However, Grousset et al. (1988) show that zonation of Nd values occurs throughout an ocean basin, reflectingvariations in provenance. McLennan et al. (1990) demonstrate that the Nd-Sr signatures of modern tutbiditesvaiy considerably, and that this variation can be related to tectonic setting. The largest degree of variation hasbeen found at active plate margins, and is the result of the lack of mixing between arc-derived marginal basinsediments and the ambient, continentally derived, oceanic sediment mass (Grousset et al., 1988; Frost andCoombs, 1989). Such variations suggest that Nd isotopic fluctuations within a marginal basin succession mayuniquely fingerprint that succession, allowing it to be compared to, or discriminated from, strata in different,coeval basins. The inference is that coeval strata within a single basin should display similar and correlatable14isotopic variations, and these variations should differ from strata in other basins as a result of variations inprovenance resulting from differences in tectonism, basement character, and magmatic episodicity within eachbasin’s source area.The purpose of this study is to assess the viability of utilizing sedimentaiy isotopic fluctuations as astratigraphic tool in basin analysis and the reconstruction and discrimination of individual basinal sequences.This study focuses on the isotopic signatures of Plio-Pleistocene sediments from marginal basins in the westernPacific. It seeks to determine the lateral and vertical variability of the isotopic signature of fine-grainedsediments within these basins. Assessment of the viability of utilizing isotopic fluctuations as a stratigraphictool requires a rigorous test case that allows direct comparison between the sedimentary isotopic signature andthe isotopic characteristics of the source terrane. Western Pacific marginal basins provide a test case whereinthe evolution of each basin is relatively well understood, as is the geology, isotopic systematics, and volcanicand tectonic episodicity of the source regions. The isotopic variability within strata from these basins cantherefore be interpreted within a well constrained geologic framework. The Nd and Sr isotopic signatures offine-grained sediments spanning the Plio-Pleistocene boundary from the Shikoku Basin in Philippine Sea, Seaof Japan, and Sulu Sea are used as a test case. Specifically, this study aims to determine: 1) if coevalsediments from geographically and geologically separate basins are isotopically distinct; 2) if isotopicfluctuations within stratigraphic sequences can be used as a stratigraphic correlation tool; and 3) the utility ofisotopic fluctuations in delineating basin evolution.2.2 BASIN DESCRIPTIONThree basins were selected for evaluation based on the quality of constraints on basin evolution andsedimentary provenance, and on the presence of sufficient drill sites within the basin. Sediment samples wereobtained from drill sites cored by the Deep Sea Drilling Project (DSDP) and the subsequent Ocean DrillingProgram (ODP; Fig. 2.1).15l000m /2000mFigure 2.1 - Geographic map of western Pacific marginal basins, with location of ODP/DSDP drill sitesexamined in this study. Highly simplified cnistal domains are shown, with average isotopic character ofmajor source regions for marginal basin fine-grained sediments. Data sources listed in text.j ODP Drill Sitebathymetric16Basins chosen for this study are the Shikoku Basin of Philippine Sea, Sea of Japan, and Sulu Sea (Fig.2.1). These basins have been the subject of Ocean Drilling Program (ODP) investigations which havedocumented the basin morphology, physical sedimentology, biostratigraphy, sedimentation rates, anddepositional setting of each basin (deVries Klein et al., 1980; Rangin et al., 1990; Tamaki, et al., 1990). Abrief synopsis of each basin follows:a. Sea ofJapanThe Sea of Japan is a complex marginal basin that was initiated in early middle Miocene time byrifting of a continental arc. The Sea of Japan separates cratonal rocks of the Eurasian plate from the Japanesevolcanic arc. It comprises two sub-basins, the Yamato Basin and the Japan Basin, which are separated bytopographic highs interpreted as foundered and rifted continental blocks (Fig. 2.1; Tamaki, et al., 1990). Thenorthern Sea of Japan contains the Japan Basin, which is a deep basin (3000-3700 m) underlain by oceaniccrust capped by Miocene to Recent sediments. Ocean Drilling Program sites 794 and 795 are located alongthe eastern margin of the Japan Basin; site 301 is in the southern end (Fig. 2.1). The Yamato Basin isshallower (2000-2500 m) and smaller than the Japan Basin, and is separated from it by the Yamato Rise, aprominent topographic high composed of Mesozoic granitoids. Site 797 is located on the western margin ofthe Yamato Basin, immediately adjacent to the Yamato Rise (Fig. 2.1)Sedimentation in the Sea of Japan was characterized by the deposition of diatomaceous hemipelagicsediment during the late Miocene and early Pliocene, and by an influx of terriginous clastic debris during thelate Pliocene and Quaternaiy. The increased clastic flux, accompanied by an increase in volcanic input,resulted from uplift of the northeast Honshu arc, which flooded both sub-basins with detritus during the latePliocene and Quaternary.Principal sediment sources to the Sea of Japan include: 1) northeast Japan “transitional” arc(Hokkaido and northern Honshu); 2) southwest Japan “continental” arc; and 3) Sino-Korean craton (Fig. 2.1).17Figure 2.2- Generalized stratigraphic columns of Shilcoku Basin, Sea of Japan, and Sulu Sea ODP/DSDP drillholes. Data from deVries Klein et al. (1980), Rangin et al. (1990), and Tamaki et al. (1990). Bracketsindicate stratigraphic interval examined; mbsf = metres below sea floor.Calcareousmud}0Clay andsilty clayAsh-richclayCaicareousmarlNannofossilooze to clayooze}Cp.Mudstone withash layersDiatomaceousclayChert andsiliceous clayClaystone withnannofossil chalk500mbsfVolanicbasementEJAClaystone andsiltstoneClay andtuffaceous clayVolamcbasementmbsfShikoku Basin Sea of JapanVolcaniclasticsediments800 -_____mbsfSulu Sea18Northeast Japan contains extensive Cenozoic volcano-sedimentary cover, including the Miocene Green Tuffbelt, overlying Mesozoic subduction-accretion complexes (Isozaki et al., 1990), and is characterized bytransitional ENd values (—+5-+6; Nohda and Wasserburg, 1981). Southwest Japan is underlain by latePaleozoic to Tertiary subduction-accretion complexes and granitic intrasions. ENd values vary fromapproximately (-6) to (+4), reflecting the influence of older continental basement (Morris and Kagami, 1989;Nakamura Ct a!., 1990; Isozaki et a!., 1990, Kagami et al., 1992). The Sino-Korean craton is primarilycomposed of Archean and Proterozoic igneous, metamorphic and metasedimentary rocks, minor Paleozoicsedimentary rocks and Mesozoic continental arc rocks (Xiong and Coney, 1985; Parfenov, and NataPin, 1985).Isotopic values reflect the cratonic affinity of the region, with87Sr/6r>0.706, and ENd values <<-15 (Xii,1990).b. Shikoku BasinThe Shikoku Basin of the Philippine Sea is an elongate, fan-shaped inactive backarc basin thatopened in the early to middle Miocene (deVries Klein and Kobayashi, 1980; Park et al., 1990). The basin islocated in the northeast portion of the Philippine Sea, and is flanked on the north by the Japanese Islands andon the east by the Izu Bourn arc. The Shikoku Basin merges to the south with the Parece Vela Basin, and bothare isolated from the rest of the Philippine Sea by the Kyushu-Palau Ridge (Fig. 2.1). The Shikoku Basin isroughly divided into two subbasins by the Kinan Seamount Chain, with the eastern portion displaying arougher and shallower topography than the western portion (Park et al., 1990). Water depth in both areasexceeds 4500 m (de Vries Klein et a!., 1980). Deep Sea Drilling Project (DSDP) drill holes 297 and 442 are inthe western subbasin; drill holes 443 and 444 are in the eastern subbasin (Fig. 2.1).Seismic profiles demonstrate that the Shikoku Basin contains three coalescing clastic wedges, whichprograded into the basin from the north, west, and east, reflecting clastic influx from Japan, the Kyushu-PalauRidge, and the Izu-Bonin arc, respectively (Karig, 1975). Pliocene sedimentation was characterized bydeposition of hemipelagic mudstone and volcanic ash (Figs. 2.2, 2.3) Abundant resedimented sandstones19442 443 4440-— - =—E: zrE IV — L...C— — UV — Q_J__-,•::—— Generalized Lithologics—clayand silty clay—— clays silts calcareous oozesiliceous clayclaystone—— calcareous mudEashy cby500— — pumaceous siliceous mudvolcanic basementmbsf—+ biostratigraphic control point*sample site(see table I for precise depths)Figure 2.3 - Stratigraphic sections of DSDP drill holes in the Shikoku Basin. Arrows indicate biostratigraphiccontrol points. Data from deVries Klein et a!. (1980). Stars indicate sample sites; precise sampleintervals are given in Table 2.1; mbsf= metres below sea floor.20occur in the northern portion of the basin (Curtis and Echols, 1980). Pleistocene sedimentation was dominatedby the deposition of hemipelagic mudstone with a high percentage of calcareous biogemc sediments. Thedecrease in terriginous clastic influx in the Pleistocene is the result of decreased volcanic activity in the IzuBonin arc, and the inception of the Nankai Trough at the northern end of the basin.The lzu-Bornn arc on the east and southwest Japan to the north are the dominant sediment sources inthe Shikoku Basin. The Kyushu-Palau Ridge to the west was intermittently emergent and supplied sediment tothe basin during the Pliocene and Pleistocene. The Izu-Bonin arc is an oceanic arc that has supplied bothvolcanic ash and volcanic clastic debris to the Shikoku Basin since the Miocene (Curtis and Echols, 1980).The arc is typified by island arc tholeiitic basalt characterized by juvenile ENd values (+8 - +9) (Nohda andWasserburg, 1981). Southwest Japan, as previously described, displays ENd values between approximately (-6) and (+4) (Morris and Kagami, 1989; Nakamura et al., 1989; Isozaki et al., 1990, Kagami et al., 1992).Isotopic data for the Kyushu-Palau Ridge are unavailable, but it is assumed to be juvenile in character.c. Sulu SeaThe Sulu Sea is a deep (4000-5000 m), elongate, restricted basin situated west of the Philippine arcand northeast of Borneo (Fig. 2.1). It is one of the smallest marginal basins in the western Pacific. The SuluSea is divided into two subbasins by the northeast-trending Cagayan Ridge. The subbasins are flanked byprominent ridges that are parallel to the elongation of the basin. These ridges are alternately covered by reefsor emergent, exposing thrust faulted continental rocks on both the northwest and southeast side of the basin(Rangin and Silver, 1990).Pliocene strata in the Sulu Sea consist of a thick sequence of turbidite sandstone, siltstone and clayoverlain by hemipelagic claystone, which is in turn overlain by Pleistocene thin to thick-bedded pelagicforaminifer-nannofossil marls, carbonate turbidites, and volcanogenic clayey siltstone (Rangin et al., 1990).Volcanic ash beds occur throughout the sequence. The general fining-upward trend is interpreted to indicate a21decrease in the importance of terriginous clastic input and an increasing importance of hemipelagic andpelagic sedimentation across the Plio-Pleistocene boundary.The Sulu Sea is a back-arc basin formed within a Neogene collision zone involving Mesozoicaccretionary prisms and the Philippine arc (Rangin, 1989). There are four principal sediment sources for theSulu Sea: 1) Mesozoic metasediments, schist, olistostrome, and ophiolite of the Palawan accretionaiy prismand underlying continental basement exposed to the northwest; 2) Philippine arc volcanics to the east; 3) Suluvolcanic ridge and underlying Mesozoic accretionary prism to the southeast; and 4) eastern Borneo, whichconsists of a Mesozoic accretionary prism (Rangin, 1989; Rangin and Silver, 1990). Isotopic data are sparsefor the Mesozoic accretionary complexes west of the Philippine arc, but tectonic models for the region indicatean affinity with cratonal materials from southern China, suggesting probable ENd values <<0 (Faure et al.,1989). The Philippine arc is typified by ENd values of (+5)-(+9), and 87Sr/6rof < 0.7060 (Defant et al.,1989).2.3 METHODSA. Sample SelectionLate Pliocene and Early Pleistocene fine-grained sediments were sampled in each basin (Fig. 2.2).Approximately the same chronostratigraphic interval was sampled in each basin in order to provide temporalconstraints on isotopic fluctuations induced by changes in seawater chemistry, climatic conditions, andeustasy. Young sediments (<4 Ma) were sampled because the modern geologic setting and basinconfiguration provide reasonably good constraints on basin morphology, sedimentologic patterns, and basinmargin tectonism and volcanism during the time of deposition. The shallow burial depth of the sediments(<250 m) limits diagenetic alterations to those imparted under shallow burial diagenesis, which should belimited to partial equilibration with seawater isotopic compositions and the deposition of authigenic cements(Singer and Muller, 1983; Hesse, 1990). The Plio-Pleistocene boundary in each basin is roughly coincident22with a significant change in sediment type, allowing the influence of provenance variations on the isotopicsignature to be evaluated.Multiple drill sites were sampled from each basin (Fig. 2.1). Drill cores were selected according to:1) the density of drill sites in the basin; 2) the location of the drill site with respect to the basin margin; and 3)the percentage of core recoveiy through the stratigraphic interval of interest. Adequate spacing of drill siteswas required to completely sample the basin and incorporate lateral sedimentologic variations throughout thebasin. Drill sites in the interior of the basins were chosen where possible to avoid proximal sediment near thebasin margin, thereby minimizing point source bias and maximizing the probability of complete hemipelagicsediment mixing. The percent core recovery was a deciding factor in selecting drill cores to be sampled; drillcores with poor recovery or excessive drilling disturbance were avoided.Sediments within individual drill cores were sampled symmetrically about the Plo-Pleistoceneboundary, which is biostratigraphically located in each drill hole, providing a control point for absolute agecontrol (Fig. 2.2). Samples were taken at 0.4-0.5 Ma intervals, based on calculated sedimentation rates, andan interval of approximately 1-1.5 Ma was sampled on either side of the Plio-Pleistocene boundaiy. Highersample frequency (0.1-0.3 Ma/sample) was utilized in the Shikoku Basin to test the viability of intrabasinalcorrelation. Samples were selected such that each lithologic unit within the sample interval was represented.An assumption inherent in this sampling procedure was that fine-grained, homogeneous sedimentaryintervals represent well-mixed hemipelagic and pelagic sediment, indicative of the ambient “background”sedimentation of the basin. Slightly coarser, graded beds, probably turbidite deposits, are considered to be ofprobable “point-source” origin, and thus were not sampled. Only relatively thick (> 20 cm), fine-grained,homogeneous clay and silty clay intervals were sampled, in order to avoid turbidity-current-relatedhemipelagic deposits. Laminated silt and fine sand intervals were avoided, as were volcanic ash layers. It isrecognized that much of the fine-grained, homogenous sediment in any deep marine basin, even within thicksequences, may be of turbiditic origin. However, the majority of this fine-grained sediment was initially23transported in fluvial or aeolian suspension, and therefore has the highest probability of representing anintegrated, “average” crustal composition of the source area.B. Age ControlThe age of sediment samples was determined by paleontologic studies of foraminifera, nannofossils,and radiolaria (de Vries Klein et al., 1980; Okada, 1980; Rangin et al., 1990). Paleontologic resolution variedconsiderably between drill holes, as a function of core recoveiy, faunal preservation, and paleontologicsampling interval. The Plio-Pleistocene boundary has been located as accurately as possible in all drill holes.(Okada, 1980; Tamaki et al. 1990). Sedimentation rates are established for each lithologic unit based on unitthickness and the estimated depositional time span; the accuracy of the calculated rate is dependent on thedegree of paleontologic and magnetostratigraphic resolution (deVnes Klien, 1980; Okada, 1980; Tamaki,1990). This method of estimating sedimentation rate implies a steady sediment accumulation through time,which is obviously not the case in active marginal basins characterized by rapid episodic sedimentaccumulation. However, in the absence of reliable isotopic chronostratigraphy, sedimentation rates are theonly method available for estimating the age of any particular sample interval. The absolute age of eachsediment sample was estimated by extrapolating the sedimentation rate for each lithologic unit from the PlioPleistocene boundary to the sample depth in question. This method is the best available means of ageestimation, although the confidence level of the age estimate is variable within each drill hole, and, in general,decreases with distance from the Plio-Pleistocene boundary.C. Analytical ProceduresSediment samples (n’70 ) were obtained from drill cores at the Ocean Drilling Program Gulf Coastand West Coast Repositories. Approximately 12 cm3 of sample were air dried and crushed to fine powder in amechanical agate mortar. Sample splits of 200-300 mg each were processed separately for Sm/Ndconcentration, and Sr and Nd isotopic composition.24a. Rb/SrSample powders underwent warm (+ 100°C) dissolution in concentrated HF in Savillex beakers. Noattempt was made to leach out the carbonate fraction prior to dissolution, with the exception of one testsample. Sr isotopic composition was measured on unspiked samples purified utilizing standard cationexchange techniques (Faure and Powell, 1972). Sr isotopic measurements were made on a VG-Isomass 54Rmass spectrometer at The University of British Columbia. Measured isotopic ratios have been normalized to a86Sr/8rratio of 0.1194. Replicate analyses of National Bureau of Standards standard SrCO3 (SRM987)during the course of this study give a value 0.000078 below the 87Sr/6rreference value of 0.710190 +1-0.000020 (n=16).Rb and Sr concentrations were determined by replicate X-Ray fluorescence analyses of pressedpowder pellets, using U.S. Geological Survey rock standards for calibration. Rb/Sr ratios have a precision of2% (1 s); concentrations have a precision of +1-5%.b. Sm/NdIsotopic dilution analysis was performed on spiked samples, whereas Nd isotopic concentration(143Nd/4)analysis utilized unspiked samples. All samples underwent a 12-hour warm predissolution inconcentrated HF in Krogh-style Teflon dissolution bombs (Krogh, 1973), followed by a 5-day dissolution inthe same bombs in a HF-FINO3mixture at 190°C to ensure dissolution of all refractory phases.For isotopic dilution analysis, a mixed 149Sm-’50Ndspike was added prior to initial dissolution(Andrew et aL, 1991). Following 5-day dissolution, the “bomb” sample was fluxed repeatedly in 6N HCI toconvert fluorides to chlorides. Rare Earth Elements (REE) were separated using standard cation exchangechromatography with 5N HC1 as the elutant. The dried REE residue was loaded onto double Re filaments, andrun on a modified VG-Isornass 54R mass spectrometer at The University of British Columbia. Replicate25analyses of international USGS standards (BCR, BHVO, BIR) and samples from the current study demonstratea concentration reproducibility of < 5%, and ratio reproducibility of < 2%.For isotopic composition analyses, the unspiked “bomb” sample was fluxed repeatedly in 6N HC1following 5-day dissolution and the group REE were purified by standard cation exchange chromatography,using 3N HC1 followed by 6N HC1 as the elutriant. Neodymium was separated from the group REE on Tefloncolumns containing Teflon powder treated with HDEHP (di-2-ethylhexyl orthophosphoric acid), using 0.25 NHC1 as elutant. The concentrated Nd was loaded onto a double Re filament assembly, and analyzed on aFinnigan MAT 261 multicollector mass spectrometer in static collection mode at the GeochronologyLaboratory of the Geological Survey of Canada (Theriault, 1990). Measured ratios are normalized to a146NW4dratio of 0.7219 (O’Nions, et al., 1979), and corrected to La Jolla Nd standard143Nd/’44d=o.5 11850. Blanks for Sm and Nd are approximately 0.1 and 2 ng, respectively.D. Data PresentationIsotopic ratios are reported as initial ratios, calculated at the depositional age of thesediments. The young age of the sediments results in very minor corrections relative to present day values.The isotopic ratio of the sediments at the time of deposition is assumed to reflect the average isotopiccomposition of the entire drainage basin at that time; the drainage basin will in most cases comprise multiplesources of different isotopic signatures. The average isotopic composition will therefore vary with time as afunction of the relative contribution of any individual source to the total sediment budget of the basin at anypoint in time. The depositional age of the sediments is younger than the actual age of the source areas;therefore, the initial ratios reported represent minimum initial isotopic values.The possibility of strontium isotopic equilibration between seawater and terriginous detritus makes itdifficult to isolate either signal individually. Carbonate cement was leached from one sample using weak (1N) hydrochloric acid in order to determine if isotopic equilibration had occurred. The leached fraction26displayed an87Sr/6r ratio higher than that of coeval seawater values (Hodell et al., 1991), suggestingpartial equilibration between seawater Sr from authigemc carbonate cement and detrital Sr. Therefore,strontium analyses are on whole rock sediment samples, and the measured 87Sr/6r ratios represent amixture of seawater Sr and detrital Sr isotopic ratios. Seawater Sr isotopic values vaiy linearly from 0.7090 -0.7092 (Hodell et a!., 1991) during the time period being analyzed for this investigation; any Sr isotopicfluctuation outside this range is interpreted to be the result of changes in the detrital signature.Neodymium analyses are expressed in initial ENd notation (DePaolo and Wasserburg, 1977), usingpresent-day values of CHUR143Nd/4d=0.512638 and 147Nd/4d=0. 1967 (Jacobson and Wasserburg,1980).2.4 RESULTSIsotopic data for the Shikoku Basin, the Sea of Japan, and the Sulu Sea are graphically presented asinitial 87Sr/6rversus initial ENd on figure 2.4 and are tabulated in Table 1. The strontium values for allsamples are between 0.7065-0.7160, and ENd values are consistently negative [(-0.5)- (-9.3)1.The Sulu Sea isotopic signature is distinct from both the Sea of Japan and the Shikoku Basin, withlower initial Sr values and less evolved (= less “continental”) Nd values. There is a high degree of overlapbetween the Shikoku Basin and the Sea of Japan, with both displaying the same range in Sr values. The meanvalue for ENd in Sea of Japan sediments, however, is slightly higher (less evolved) than coeval sediments inthe Shikoku Basin. The more evolved values in the Shikoku Basin were unexpected, in that the Shikoku Basinis isolated from any obvious sources of evolved Nd. In both the Sea of Japan and the Shikoku Basin, there is aroughly inverse relationship between 87Sr/6rand ENd. This relationship is not apparent in the Sulu Sea,but this is probably the result of the smaller sample set. There is no consistent linear relationship betweeneither87Sr/6rvalues or ENd values and the age of the sediments in any of the basins.270.0087SrI 86Sr VS. S Nd•.2.00 - Shikolai Banii (a’rage). A Seaoflapan•Sea of Japan (average)A SiihiSea4.00. •_____________A A600A+--A Avg.ErrorA A-8.00Ad.•A A •A- 0.780 0.7100 0.7120 0.7140 0.7160 0.718087SrI86SrFigure 2.4 - 87SrI6rversus £Nd values for all samples studied. Note strong degree of overlap betweenShikoku Basin and Sea of Japan, contrasting with separation of Sum Sea samples.28Depth(mbsfAge(Ma)Sr(m)SrRb8lRb/86SrSr(i)SmppmNdppmml4lINdl4143/144EpsilonBpsilon(i)SEAOFJAPAN3014-5161.281.390.711233121.13130.053.110.711176.0229.600.12300.5123855-0.3747-4.94-4.92301-5-6182.201.640.713513129.43135.323.030.713446.1031.000.11920.5123035.0.3940-6.53-6.52301-7-1213.452.000.713843121.46135.223.220.713755.9130.400.11750.5122525-0.4026-7.53-7.51301-8-1240.592.320.71601487.74107.283.540.715904.6723.280.12130.5121885-0.3833-8.78-8.76794-5-542.741.210.711674163.02107.611.910.711637.1633.860.12790.5123017-0.3498-6.57-6.56794-7-155.541.640.713103137.19130.862.760.713036.3731.000.12420.5122915-0.3686-6.77-6.75794-8-672.712.320.712103126.0198.692.270.712034.5222.450.12170.5122958-0.3813-6.69-6.67794-10488.382.790.71318484.5276.522.620.713083.9919.330.12480.5123118-0.3655-6.38-6.35‘794-11-294.052.960.711553132.2383.181.820.711484.8122.490.12940.5123135-0.3421-6.34-6.31i794-11-6101.203.140.712987123.15100.382.360.712875.7127.270.12660.5122964-0.3564-6.67-6.64795-7-563.481.050.709195145.0573.251.460.709174.6520.870.13460.51246214-0.3157-3.43-3.42.795-9-‘795-16-1143.132.140.70964682.9172.012.160.709353.9417.800.1337%Q797-7-458.591.130.715454127.95125.092.830.715406.9234.600.12100.5122156-0.3849-8.25-8.24797-9-579.291.580.711558321.27117.191.060.711535.5426.590.12680.5122186-0.3554-8.19-8.18797-11-6100.242.010.712398173.49115.161.920.712345.6526.940.12670.5122266-0.3559-8.04-8.02797-13-5117.772.360.712243110.9984.382.200.712170.5123284-0.3645-6.05-6.03797-15-6137.812.760.713573140.98123.012.160.713484.2419.490.13140.5122174-0.3320-8.21-8.19Average0.71194-6.35SULUBASIN768-11-291.23635.2833.710.150.000000768-134113.04798.8633.920.120.000000768-14-1119.17843.2040.020.140.000000768-14-5124.17151.39100.191.910.000000768-15-6135.480.707494167.6986.861.500.707494.1619.510.12920.5124396-3.88768-16-6145.310.705449111.94102.912.660.705440 0769-10-385.060.708613644.5434.260.150.708612.169.630.13580.5126156-0.45769-11-293.070.708614674.1640.430.170.708610.51248224-3.04769-12-2102.630.707133206.4171.851.010.707133.8616.930.13840.5125004-2.56769-13-5116.810.707953182.0986.671.380.707954.7120.650.13730.51241884.29769-14-3123.210.7075031178.0665.420.810.707504.0718.660.13210.5124847-3.00771-IR-1100.81405.4271.910.510.00000Depth(mbsfAge(Ma)Sr(m)SrRbSlRb/S6SrSr(i)SmppmNdppmm147/Nd14143/144eN0)eNd(i)SHIKOKUBASIN442-12-5112.531.290.712773118.42114.702.800.712725.1225.470.12170.5122775-0.3813-7.04-7.03442-13-2119.001.520.713403123.22105.172.470.713355.5428.030.12020.5122516-0.3889-7.55-7.53442-14-3129.961.850.71346Ii125.9761.782.570.713406.7929.700.13820.5122627-0.2974-7.33-7.32442-15-2137.402.090.7129310127.60104.302.360.712865.7327.350.12670.5122689-0.3559-7.22-7.20442-16-2146.892.400.715723115.96122.993.070.715626.0729.570.12420.51221212-0.3686-8.31-8.29442-18-1164.582.970.713055123.54113.112.650.712945.8929.310.12270.5122848-0.3762-6.91-6.88442-18-3167.503.060.7102218139.6199.272060.710136.4729.080.13450.5123717-0.3162-5.21-5.18442-19-2175.603.330.713793128.10122.422.770.713667.3633.350.13340.5122406-0.3218-7.76-7.74443-11-295.381.430.711255253.72116.631.330.711225.4024.320.13410.5122408-0.3183-7.76-7.75443-11-6100.201.490.714399120.50131.873.170.714326.1128.820.12820.5122164-0.3782-8.23-8.22443-12-2104.101.540.712013165.78114.772.000.711974.9224.040.12380.51220713-0.3706-8.41-8.39443-14-2123.841.900.712494161.45120.382.160.712435.2725.490.12500.51234616-0.3645-5.70-5.68.443-14-4126.542.020.711234121.20118.982.840.711145.6825.830.13290.5123249-0.3244-6.13-6.11443-15-2133.192.330.713815123.94127.132.970.713715.8429.440.12080.5122459-0.3859-7.67-7.64443-15-4135.882.450.715084128.48127.052.860.714986.1329.670.12500.5126448-0.3965-9.01-8.99443-15-6139.022.590.713836124.80127.062.950.713725.8229.400.11980.5122387-0.3910-7.80-7.78443-16-2143.802.810.713244138.30121.622.540.713145.8427.580.12830.51232723-0.3477-6.07-6.04443-17-1151.853.180.712743138.52157.073.280.712595.4327.340.12040.5122489-0.3879-7.61-7.58443-18-1161.853.640.711964132.72115.872.540.711826.2428.850.13060.5122905-0.3360-6.79-6.76444-4-531.230.450.709752182.8468.591.090.709744.7120.410.13930.5124525-0.2918-3.63-3.63444-5-137.000.690.712444146.22116.602.310.712416.2029.060.12810.5122867-0.3488-6.87-6.86444-5-441.600.880.712582148.72119.752.330.712555.5428.150.11910.5122818-0.3945-6.96-6.96444-6-148.921.220.711143209.89110.731.530.711115.2225.990.12160.5122996-0.3818-6.61-6.60444-7-154.741.750.712155148.41111.802.180.712106.2129.860.12560.51219610-0.3615-8.62-8.61444-7-357.321.980.710564144.54106.932.140.710505.6327.420.12270.51234425-0.3762-5.74-5.72444-7-661.232.330.710903152.36103.691.970.710846.2329.140.12910.5122549-0.3437-7.49-7.47444-8-165.882.750.707692220.3673.120.960.707655.2222.110.14300.51252315-0.2730-2.24-2.22444-9-177.153.770.709383164.5999.731.750.709295.4029.010.12480.51234510-0.3655-5.72-5.68444-1-lA82.754.270.711402141.25106.142.170.711275.8027.700.12680.51229412-0.3554-6.71-6.67297-6-583.490.850.712123140.73117.402.410.711845.4027.370.11770.51223613-0.4016-7.84-7.83297-7-3100.101.010.713973144.45128.102.570.713935.7027.940.12480.5122392-0.3655-7.78-7.77297-8-4110.101.120.713544140.50123.852.550.713505.5027.360.12140.5122516-0.3828-7.55-7.54297-9-2127.101.290.714565117.22133.733.300.714505.4028.000.11670.5122336-0.4067-7.9-7.89297-10-2154.701.560.712803119.53114.542.770.712745.2927.310.11700.5122767-0.4052-7.06-7.05297-11-2203.392.050.712983125.29118.332.730.712904.9324.860.12000.512276-0.3899-7.06-7.04297-11-4206.332.080.711693176.8662.681.840.711645.7725.870.13490.5122636-0.3142-7.32-7.30297-12-2.249.952.520.712773120.34122.802.950.712665.5227.260.12250.5122498-0.3772-7.59-7.56Average0.71235-6.99The relationship between Sr and Nd isotopic values and stratigraphic position is detailed in Figs. 2.5-2.7. These figures portray isotopic fluctuations with depth and age for three drill holes in the Shikoku Basin(ODP site 442, 443, 444; Figs. 2.1, 2.3). Neodymium values fluctuate by up to 5 ENd units in any one drillhole. Examination of depth versus 8Nd indicates that similar isotopic patterns occur in eveiy core, althoughat different depths (Fig. 2.5). Each graph in figure 2.5 shows two peaks characterized by shifts to morepositive values, and two troughs, characterized by shifts to more negative values. Superimposing these graphson one another suggests that similar isotopic variations occur at different depths in each drill hole; thesedifferences in depth may be related to differences in sedimentation rates in different portions of the basin(deVries Klein et al., 1980). Plotting ENd versus age (Fig. 2.6) for the same drill cores illustrates that theisotopic fluctuations appear to occur in all cores within a narrow time range. For example, the strongestpositive shift in core 444 occurs between 2.35 and 2.75 Ma (65.88 metres below sea floor [mbsfj), and may becorrelated with less dramatic shifts in core 443 at 2.81 Ma (143.8 mbsf) and core 442 at 3.06 Ma (167.5 mbsf).Strontium values range from 0.7080-0.7160 in the Shikoku Basin, and the maximum range in87Sr/6r ratios in any core within the stratigraphic interval studied is approximately 0.005 (Fig. 2.7).Strontium ratios are consistently higher (up to 0.006 higher) than ambient seawater values for this time period(Hoddel, 1991). Variations in 87Sr/6r ratios in the Shikoku Basin are roughly inversely correlated withvariations in ENd values (Figs. 2.6, 2.7). A strong increase in Sr initial ratios occurs between 3.0 and 2.5Ma, followed by a significant decrease in Sr initial ratios between 2.50-2.00 Ma. This increase in Sr initialratios corresponds to a decrease in ENd during the same time period, and the subsequent decrease in Sr initialratios corresponds to an increase in ENd31-2.00 -1442—10.0o_I I4) 80 120 160 200depth (mbsf)-2.00I-4.004.0040 80 120 160 200depth (mbsf)- 444Nd-10.00. II4() 80 120 160 200depth (mbsf)Figure 2.5- Plots of depth (metres below sea floor) versus njj for Shikoku Basin drill cores. Heavy dottedline is Plio-Plejstocene boundaiy. Note boundary represents inflection point in isotopic values in all drillholes.32Shikoku Basin-2.00I II Agevs.6442-4.00.-6.00. 4, 4,-8.00-10.00 I0.00 2.ôC) 3.00 4.00Age (Ma)Figure 2.6 - Plot of age versus d for Shikoku Basin samples. Note similarity of inflection points in all drillholes. Isotopic fluctuations are sharp, and occur in a relatively confined time span. Arrows indicateapparently correlative horizons between sections.33• Shikoku Basin0.7160- +I IAge vs. Sr87/Sr860.7140 If 442—443—A--0.7120Cl)0.7100.0.7080.0.7060.0.00 2.b0 3.bO 4.bOAge (Ma)Figure 2.7 - Plot of age versus87SrJ6rfor Shikoku Basin samples. Note similarity of inflection points in alldrill holes. Compare to figure 2.6, noting inverse relationship for Nd and Sr isotopic values in coevalsamples.342.5 DISCUSSIONA. Interbasinal Isotopic SignaturesThe isotopic signatures of fine-grained sediments may be unique indicators useful in basindiscrimination and stratigraphic correlation. The isotopic signature of fine-grained Plio-Pleistocene sedimentsin Western Pacific marginal basins varies substantially, but values within any individual basin are confined toa relatively limited range (Fig. 2.4). The range of values in any one basin is the result of the geology andgeologic evolution of the source region. The geologic setting of each marginal basin is unique, and thereforethe isotopic signature has potential for use in the discrimination of individual basins. McLennan et al. (1990)point out that back arc basin sediments vaiy petrographically, geochemically, and isotopicafly due tovariability in provenance, particularly the availability of old continental crustal material. They differentiatebetween SW Pacific (Celebes and South China basins), Aleutian, and Japan basins based on variablepetrographic and geochemical signatures.Different crustal domains have different average isotopic values, and basins adjacent to these domainswill have signatures characteristic of the domains on its margins. Cmstal domains may be broadly groupedinto: 1) old continental material ( Ej < -6); 2) transitional crust ( Ej between -6 and +5); and 3) juvenilecrust ( E >+5; DePaolo et al., 1991; DePaolo, 1981). The isotopic signature of a basin represents variabledegrees of mixing between basin margin domains and ambient seawater values, as reflected by intrabasinalcaibonate. Fluctuations in mixing parameters through time will alter the isotopic signature, but within arelatively limited range defined by the isotopic range in the source regions.In the present study, the absolute isotopic values vary between 8Nd(0)- (-9), and87Sr/6r=(0. 060) - (0.7 160). Samples with higher Sr values (>.7100) and lower eNd values (< -7.00)indicate the increased importance of old continentally derived material; samples with lower Sr values35(<0.7 100) and higher ENdvalues (>—6.00) indicate increased contamination by transitional and juvenilecrustal material.The isotopic values of each basin, with the exception of the Shikoku Basin, approximate anticipatedvalues. The Sulu Basin has Nd values between 0 and 4.25, suggesting a mixing of carbonate sediment(probable Nd of -4 to -2 (Piepgras and Wasserburg, 1980)) with juvenile arc material (+5-+9) derived fromthe Philippine arc, together with very minor amounts of detritus from Mesozoic accretionaiy prisms and minorunderlying continental basement (transitional to evolved uNd values). This interpretation is consistent withthe sedimentary petrology described by Rangin et al. (1990). The low87Sr/6rvalues in the Sulu Basin alsoreflects the high influx of carbonate debris from flanking reef complexes (Rangin et al., 1990).Sediments from the Sea of Japan are characterized by ENd values between (-3) and (-9), and87Sr/6r=(0. 090) - (0.7160). The Nd values are strongly skewed to the more negative end of the range(Fig. 2.4), reflecting the contribution of cratonal detritus derived from the Sino-Korean craton to the north andwest ( 8Nd = << -15), and the rifled continental block of the Yamato Rise ( ENd = << -3; Nakamura et al.,1990). This continental detritus is strongly diluted by volcanogenic material derived from both the southwestJapan “continental arc” ( Nd =(-6)-(+4); Morris and Kagami, 1989; Kagami et al., 1992) and the northwestJapan “transitional arc” ( ENd =(+5)-(+6); Nohda and Wasserburg, 1981). This dilution is supported by bothsedimentary petrology of associated sandstones (Marsaglia et al., 1992), and by mixing models that indicatethat only a minor amount of cratonally-derived sediment needs to mix with arc-derived detritus to significantlylower the ENd value, due to low Nd abundance in juvenile cmstal detritus and very negative Nd values ofolder continental crust. The87Sr/6r ratio is also skewed to the higher end of the range, supporting amixing of cratonally-denved material and arc detritus.The isotopic signature of Shikoku Basin sediments strongly overlaps with the signature of those fromthe Sea of Japan (Fig. 2.4). The Nd values range from (-2) to (-9), but are skewed to the more negative endof the range, similar to the Sea of Japan values. However, Nd values in the Shikoku Basin are generally36lower (more evolved) than those in the Sea of Japan, at the same value of87SrI6. Similarly, when any oneeNd value is considered, the87Sr/6rratio of the Shikoku Basin is generally lower than the correspondingratio in the Sea of Japan. The lower Sr values were anticipated, due to the high influx of continentally derivedmaterial in the Sea of Japan, but the lower Nd values were not. These values are problematic, in that theShikoku Basin is supplied with juvenile sediment from the Izu-Bomn arc ( 6Nd =(+8)-(+9); Nohda andWasserburg, 1981) and with transitional sediment from southwest Japan ( Ej =(-6)-(+4); Morris andKagami, 1989; Nakamura et al., 1990; Isozaki et aL, 1990; Kagami et al., 1992). Carbonate detrituscontributes ,jvalues around (-4)-(-2) (Piepgras and Wasserburg, 1980). Adinixtures of these end memberswould be expected to produce sediment with ENd values of approximately (-2) to (+5), obviously dependenton the mixing ratio, yet the values in the Shikoku Basin ( ENd= (-2)-(-9)) are considerably lower thanexpected. There is no old cratonal material adjacent to the Shikoku Basin, nor is any suggested at depth byisotopic systematics (Nohda and Wasserburg, 1981; Morris and Kagami, 1989; Nakamura et al., 1990; Isozakietal., 1990).A plot0fS1U/Nd vs. E for samples from the Sea of Japan and the Shikoku Basin illustrates thestrong degree of overlap in isotopic values between the two basins (Fig. 2.8). The value uu/Nd is acomparison of the‘47Sm/144Ndof the sample against the‘47Sm/1’Nd of chondritic meteors (/S!flJNd =(4Sm/Nd)sample/’m/’NdCWJR- 1; Shirey and Hanson, 1986) that effectively measures the degreeof light rare earth element enrichment of the sample. Samples from both basins show a very limited range ofpSm/Nd values, and plot within the range expected for modern marginal basins sediments (McLennan andHemming, 1992). This restricted range indicates little or no REE fractionation has occurred during thesedimentary cycle. The samples form a well-defined mixing line between compositional and isotopic fieldsdefined for arc rocks and Precambrian upper crust, highlighting the importance of continentally derivedsediment throughout the Plio-Pleistocene evolution of the basins.The most plausible explanation for the anomalous values in Shikoku Basin sediments is an aeoliansediment flux from cratonal materials on the Asian mainland. Studies have shown an order of magnitude37Figure 2.8- Plot of1Sm/Nd vs. Nd for samples from the Sea of Japan (triangles) and Shikoku Basin(squares). The data illustrate the strong degree of overlap between the two basins, and the limited rangein isotopic values and1Sm/Nd for the samples. Note the well-defined mixing lines betweenPrecambrian crust and arc rocks formed by the data array. MORB mid-ocean ridge basalt.0.200.00-0.20-0.40-0.60I. Shikoku Basin IA Sea of Japan:-___4 <L- 4<1 --1 4-.%J t\j t’<\‘<IS4<I4<1 4---1 4--- ‘ ISArcRocksA <1- 4<. 1 4---1 4--J ISj 1’-E <4 4<7 4-. 4-..:pw%/i < IS < IS4 <L 4<PrecambrianUpper Crust14--40 -20 0Epsilon Nd (i)38increase in aeolian dust flux to the Pacific basin in Pliocene time due to increased intensity of atmosphericcirculation associated with high-latitude cooling (Janecek and Rea, 1983; Rea et al., 1985). Much of thisaeolian detritus was derived from the China-Gobi Desert aeolian source area (Janecek and Rea, 1983). andwould be characterized by cratonal Nd values ( E,jj << - 15; Rea, 1993). This material would have beentransported by the prevailing westerlies over both the Sea of Japan and Shikoku Basin, but may have beenpreferentially deposited in the Shikoku Basin due to changes in atmospheric circulation associated with thehorse latitudes at 30°N. The addition of cratonal affinity aeolian detritus would account for the low ENdsignature of the Shikoku Basin sediment, as well as accounting for the strong degree of overlap between it andthe Sea of Japan. The influx of a moderate amount of cratonal dust would have strongly altered the Nd signalwithout substantially altering the Sr values in the Shikoku Basin, which are dominated by low 87Sr/6rratios derived from juvenile volcanics and reef carbonates associated with the Izu-Bonin arc. In addition,sediments in the Plio-Pleistocene Shikoku Basin were deposited slightly above the carbonate compensationdepth (CCD; deVries Klein et al., 1980), whereas sediments in the Sea of Japan were largely deposited belowthe CCD (with the exception of ODP site 797; Tamaki et al., 1990), which would preferentially elevate the87Sr/6r ratio without affecting the ENd value in the Sea of Japan.B. Intrabasinal Isotoyic SignaturesNeodymium and strontium isotopic signatures measured in the study display significant variationswithin vertical stratigraphic sections (Figs. 2.5-2.7). These fluctuations appear to be temporally controlled,basin-wide phenomena, and are potentially useful for correlations among stratigraphic sections throughout thebasin (Figs. 2.5-2.7). Fluctuations in isotopic values reflect changes in the sediment flux in the basin,ultimately controlled by changes in the relative contribution from particular sediment sources. Isotopicfluctuations may therefore also prove useful in constraining basin evolution.ENdvalues span a range of more than five ENd units in the Shikoku Basin, and87Sr/6r ratios varybetween 0.7075 and 0.7125. Fluctuations within this range are sharp, and appear to reflect major changes in39the isotopic character of the basin in limited periods of time. Fluctuations are roughly synchronous in all drillholes, generally occurring within 0.3-0.5 Ma of each other. Although the size of the sample set precludesrigorous statistical analysis, there appears to be good correlation in isotopic variation between drill holes, asmajor inflection points in ENd values occur at about 1.5-1.6 Ma, 2.0 Ma, 2.5 Ma, and 2.6-2.8 Ma (Fig. 2.6).The strongest correlation occurs at inflection points at 1.5-1.6 Ma, 2.0 Ma, and 2.5 Ma; weaker correlations inolder and younger strata may reflect age uncertainties, which increase with depth from the Plo-Pleistoceneboundaiy, or poor biostratigraphic resolution, particularly in hole 444 (Fig. 2.3). However, the magnitude ofthese fluctuations is large enough, and the fluctuations are frequent enough to generate distinctive patterns inthe isotopic record of each stratigraphic section, which appear to be correlative across the basin (Figs. 2.5-2.7).The generally synchronous variations in these stratigraphic sections occur over approximately 5,400 2within a topographically complex basin, indicating that intrabasinal isotopic correlations are quite viable.Variations in ENd values in these drill holes are inversely proportional to variations in Sr values, providingadditional support to suggested correlations. These fluctuations may be distinct enough to allowbiostratigraphically unconstrained stratigraphic sections to be correlated with sections with goodbiostratigraphic control by correlation of the isotopic patterns.The potential applicability of isotopic fluctuations in a stratigraphic succession to provenance studiesis extremely promising. For example, the strong decrease in ENd values and increase in Sr values between2.8 and 2.5 Ma in all drill holes likely reflects a cessation of volcamc activity during the late Pliocene, and anincreasing importance of “continentally derived” material during this time. This trend ends abruptly at about2.4 Ma, when there is a sharp increase in ENd values, and an decrease in Sr values, which reflects theinitiation of volcanism in the Izu-Bonin arc. Arc-derived hemipelagic sediment flooded the basin, andoverwhelmed continentally derived input. This same line of reasoning can be used to suggest a volcanic eventbetween about 3.5 and 2.8 Ma, cessation of volcanism at about 1.8 Ma, and renewed volcanism at about 1.5-1.6 Ma. This interpretation of volcanic episodicity vs. continental sediment influx is in general agreementwith variations suggested by detailed studies of tephra and clay mineralogy within the basin (deVries Klein et40al., 1980). However, isotopic variability permits a higher resolution, is not subject to diagenetic modifications,and is not dependent on macroscopic characteristics such as grain size and colour (McLennan et al., 1991).The range in isotopic values in any basinal sequence is a function of the dynamic evolution of the sourceregion, and the absolute value of any stratigraphic level reflects a unique mixture of the different sourcessupplying the basin. Detailed analysis of the isotopic patterns in a basinal sequence should therefore provideconstraints on the changes in provenance through time, allowing for detailed reconstructions of basinevolution.2.6 CONCLUSIONSThe isotopic signature of fine-grained sediments from western Pacific marginal basins has beenexamined to test the applicability of isotopic fluctuations to basin discrimination, stratigraphic correlation, andprovenance studies. The Sulu Sea, Shikoku Basin, and Sea of Japan display isotopic signatures that varywithin limits defined by the geology of the source regions. The Sulu Sea contains a distinct signature, whereasthe Sea of Japan and Shikoku Basin display strong overlap. The Shikoku Basin displays an isotopicallyevolved signature that could not have been derived from the crustal domains on its margins, and requires inputfrom a previously unrecognized continental aeolian source. The influx of aeolian detritus in the ShikokuBasin coincides with an increase in aeolian flux to the northern Pacific associated with the onset of globalglaciation (Rea et al., 1985).Stratigraphic fluctuations in the isotopic signature of fine-grained sediment may potentially be usefulfor correlating strata basin-wide, and may provide constraints on the geologic histoiy of a region.Stratigraphic sections in the Shikoku Basin display roughly synchronous fluctuation in ENd and87Sr/6rvalues, interpreted to reflect episodicity in basin margin volcanism. Isotopic variability within stratigraphicsections has been used to correlate drill holes from across 5400j2 of basin floor.41The combined ENd-87Sr/6rsignature of fine-grained sediments appears to be useful forfingerprinting geographically separated marginal basins. The isotopic signature of a basinal sequence isprimarily a function of the average isotopic composition of all the crustal domains flanking a depositionalbasin. Thorough mixing of fine-grained sediments homogenizes the isotopic signal, and may provide asignature unique to an individual basin. Overlap of basinal signatures suggests similar provenance betweenbasins, similar provenance’s which may not be readily evident by traditional sedimentologic techniques.Although the isotopic composition of fine-grained sediments approximates the average crustal composition ofthe source region, this average crustal composition will vaiy within limits defined by the geology of the sourceregion, reflecting changes in the sediment flux derived from different domains within the source region.These temporally controlled compositional fluctuations impart a temporally controlled isotopic variation thatmay be used as a stratigraphic marker to correlate strata basin-wide. In addition, these isotopic variations arecontrolled by geologic events in the source region, and may therefore be very useful in reconstructing basinevolution. The isotopic signature of fine-grained sedimentary rocks may be an extremely useful tool indiscriminating ancient, structurally disrupted basinal sequences, and placing constraints on basin history.42CHAPTER 3REGIONAL GEOLOGIC SETTING433. REGIONAL GEOLOGIC SETTINGThe southern Canadian Cordillera comprises a number of discrete, fault-bounded tectonostratigraphicassemblages, each with a distinct stratigraphy that differs from that of adjacent assemblages. Thesetectonostratigraphic assemblages are referred to as terranes, and include, from west to east, Wrangellia,Harrison, Cadwallader, Bridge River, Methow, Cache Creek and Quesnellia (Fig. 1.2; Table 1.1). Eachterrane is defined by a stratigraphic suite that is lithologically distinct and regionally consistent, and is noteasily correlated with the stratigraphy on adjacent terranes (Table 1.1). On a broad scale, each of theseterranes consists of Paleozoic to Early Mesozoic oceanic or volcanic arc rocks, overlain by Jurassic andCretaceous island-arc volcanic rocks and volcaniclastic sediments or ocean floor assemblages. Boundariesbetween the terranes are interpreted to be Cretaceous or Tertiary regional fault systems or Jurassic to Tertiaryplutons (Wheeler and McFeeley, 1991; Monger and Journeay, 1992). In this chapter, the regional geologicsetting of the southern Canadian Cordillera is reviewed to provide context for the stratigraphic analyses thatfollow. Terrane linkages are particularly important in this investigation, and currently established linkagesare discussed for each terrane.Intense Mesozoic and Cenozoic compressional, translational, and extensional deformation andwidespread, multiphase plutonism imposed on tectonostratigraphic terranes of uncertain paleogeographicaffinity has resulted in an extremely complex geologic setting for the southern Canadian Cordillera. Anyinterpretation of Middle Jurassic basin evolution and/or paleogeographic reconstruction in the region requiresunraveling the structural, plutomc and metamorphic overprint prior to evaluating the original stratigraphicrelationships among the terranes.4492Figure 3.1- Geographic map of southwestern British Columbia highlighting locations discussed in text.453.1 TERRANE DESCRIPTIONSA. WrangelliaWrangellia is the westernmost terrane in the southern Canadian Cordillera, and is contained withinthe morphogeologic Insular Belt. It underlies the majority of Vancouver Island, as well as portions of the GulfIslands and coastal mainland British Columbia (Fig. 1.2). Wrangellia consists of Upper Paleozoic volcanic arcand carbonate platform rocks of the Sicker and Buttle Lake groups unconformably overlain by a thicksuccession (> 6 1cm) of Middle to Upper Triassic Karmutsen Formation tholeiitic basalt and associatedsedimentaiy rocks (Muller, 1977, Jones et al., 1977). Lower to Middle Jurassic volcanic arc rocks of theBonanza Group unconformably overlie the Karmutsen Formation, and are overlain by Upper Jurassic to UpperCretaceous clastic sedimentary rocks of the Kyuquot Formation and Nanaimo Group (Monger and Journeay,1994; Mustard, 1994). Early to Middle Jurassic Bowen Island Group volcanic rocks, correlated with theBonanza Group, are overlain by Cretaceous clastic rocks along the eastern margin of Wrangellia (Friedman etal., 1990).The eastern boundary of Wrangellia is interpreted as an intrusive contact with Middle Jurassicplutons of the Coast Plutonic Complex, and is exposed on Quadra Island, in the Strait of Georgia (Nelson,1979; Monger and Journeay, 1994; Fig. 1.2, Plate 1). The eastern edge of Wrangellia and the western edge ofthe adjacent Harrison terrane are both intruded by Middle Jurassic (< 167 Ma) plutons of the southern CoastPlutomc Complex, suggesting that the two terranes formed a single crustal block since at least that time(Friedman et al., 1990; Monger and Journeay, 1994). The timing of accretion of this cmstal entity withterranes to the east is uncertain (Monger et al., 1982; van der Heyden, 1992). The western edge of Wrangelliais the present-day accretionary wedge of the Cascadia subduction zone (Varsek et al., 1993).46PLUTONIC ROCKSI I Tertiary plutonsundivided metamorphic rocksSTRATIFIED ROCKS-Neogene volcanic rocksKamloops Group (Eocene)I J Nanaimo Group (KT)Gambier Group (K)I I Spences Bridge Group (K)Jackass Mountain Group (K)WI Harrison Lake Fm (lmJ)Ladner Group (lmJ)Bonanza Group (lmJ)j Shuksan Group (J)Cadwallader Group (Tr)-Nicola Group (Tr)Karmutsen Formation (Tr)Bridge River Complex (Pz-J)Cache Creek Group (Pz-J)I I Devonian sedimentary rocksIlate Cretaceous plutonsmid-Cretaceous plutonsJuraCretaceous plutonsmiddle Jurassic plutonsearly Jurassic plutons1120°—51°I0S‘SI200ill120°Figure 3.2 - Simplified geologic map of the southern Canadian Cordillera.Modified from Wheeler and McFeely (1991). Legend refers to units discussed in text.1B. HarrisonThe Harrison terrane is located in the southeastern Coast Belt, and is exposed primarily on the westside of Harrison Lake (Figs. 1.2, 2.1, Plate 1). The stratigraphic base of the Harrison terrane consists ofMiddle Triassic (Ladinian) greenstone, chert, greywacke, and minor limestone of the Camp Cove Formation.The Camp Cove Formation is unconformably overlain by Lower to Middle Jurassic (Toarcian(?) to Bajocian)arc volcanics, volcaniclastic rocks, clastic sedimentary rocks and minor conglomerate of the Harrison LakeFormation (Arthur, 1987; Arthur et al., 1993; Fig. 3.2). The Middle Jurassic arc sequence is disconformablyoverlain by fine-grained clastic rocks of the Upper Jurassic Mysterious Creek Formation (Callovian) and theoverlying volcaniclastic Bilihook Creek Formation (Lower Oxfordian). Upper Jurassic strata areunconformably overlain by Lower Cretaceous conglomerate of the Peninsula Formation (Lower Berriasian toLower Valanginian) and overlying volcanic and volcaniclastic rocks of the Brokenback Hill Formation (LateValanginian to Middle Albian; Arthur, 1987; Arthur et al., 1993).The western boundaiy of the Harrison terrane is an intrusive contact with Middle Jurassic plutons ofthe Coast Plutonic Complex; the eastern boundary is the Harrison Lake shear zone, a Late Cretaceous toTertiary dextral transcurrent fault system (Journeay and Csontos, 1989). The Harrison Lake shear zoneseparates weakly metamorphosed (subgreenschist grade) rocks of the Harrison terrane from penetrativelydeformed, highly metamorphosed (mostly amphibolite grade) imbricate thrust nappes of the Coast Belt ThrustSystem of the eastern Coast Belt (Journeay and Csontos, 1989; Journeay and Friedman, 1993; Plate 1).Middle Jurassic plutonic rocks intrude both the western edge of Harrison terrane and the eastern edgeof Wrangellia. Linkages between Harrison terrane and terranes to the east (Bridge River, Cadwallader, andMethow terranes) are uncertain, and are the focus of this study. Metasediments imbricated in the LateCretaceous Coast Belt Thrust System, including the Slollicum Schist and Twin Island Group, are interpretedto be metamorphosed equivalents to the Harrison terrane (Journeay and Friedman, 1993). This suggests that48arc successions of the Harrison terrane fonnerly extended north and east of the Harrison Lake Shear Zone, buttheir original extent is unknown.The Harrison terrane is in fault contact with the Chilliwack terrane to the south. The contact isinferred to be a high-angle fault in southern British Columbia, but Tabor (1994) documents a low-angle faultcontact between the rocks of the Harrison terrane and the overlying Chilliwack terrane in Washington. TheChilliwack terrane consists of Devonian to Permian arc volcanics, limestone, and associated clasticsedimentaiy rocks of the Chilliwack Group and overlying Upper Triassic to Lower Jurassic fine-grainedsedimentary rocks of the Cultus Formation (Monger, 1966; Monger and Journeay, 1994; Tabor, 1994).Schwagerinid fusulinid-bearing limestone clasts in the basal conglomerate of the Harrison Lake Formationhave been interpreted to be derived from the Chilliwack Group, suggesting a stratigraphic link between thetwo in the Early Jurassic (Arthur, 1987; Arthur et al., 1993).C. Bridge RiverThe Bridge River terrane is exposed in the southeastern Coast Belt, east of Harrison Lake and west ofthe Fraser River (Fig. 1.2, 3.1, Plate 1). The terrane includes rocks of the Bridge River Complex west of theFraser fault, and rocks of the Hozameen Group east of the fault (Fig. 3.1, Plate 1). Thick successions ofstructurally disrupted greenstone, interbedded chert, fine-grained clastic rocks and minor limestone dominatethe Bridge River terrane (Potter, 1983; Haugerud, 1985; Schriazza et al., 1990). The Bridge River Complexmay be subdivided into two distinct melange belts with an intervening coherent sequence (Journeay, 1993).The easternxnost melange belt, exposed directly west of the Yalakom Fault, contains abundant ultramafic rocksand serpentinite pods, including the Shulaps ultramafic complex (Calon et aL, 1990). Rare blueschist yieldsTriassic40Ar/39rages, with no record of any younger thermal perturbation (Archibald et al., 1990). Thewestern melange belt consists of ultramafic rocks and serpentinite of the Bridge River Complex. These rocksare interleaved with similar rocks of the Bralorne-East Liza complex along the west-vergent Bralorne Faultsystem, which records late Early Cretaceous defonnation (Journeay et al., 1992). Between the two melange49zones, the Bridge River Complex is a coherent sequence of interlayered greenstone, chert and argillite,conformably overlain by fine-grained clastic rocks of the Cayoosh assemblage (Journeay and Northcote, 1992;Mahoney and Journeay, 1993).Radiolarian biostratigraphy indicates an age range of Mississippian to late Middle Jurassic(Callovian) for the Bridge River Complex (Cordey and Schriazza, 1993). The Hozameen Group yieldsradiolaria of Penman to Middle Jurassic (Bajocian) age (Haugerud, 1985). Conodonts recovered fromhmestone pods within the Bridge River Complex are primarily Late Triassic in age. The age and depositionalhistory of the Cayoosh assemblage is discussed in more detail in chapter 5. Jurassic and older strata areoverlain by the Lower Cretaceous Taylor Creek Group, a coarse clastic succession of conglomerate, sandstone,and minor siltstone (Jeletzky and Tipper, 1968; Garver, 1992).The Bridge River terrane is complexly imbricated along north and northwest-trending compressional,transcurrent and extensional fault systems of Late Early Cretaceous to Tertiary age (Schiarizza et al., 1990;Coleman and Parrish, 1992; Journeay and Friedman, 1993). The westernmost boundary of structurallycoherent Bridge River strata is the Central Coast Belt Detachment, although correlative high-grademetamorphic rocks (e.g. Cogburn Group) are contained in thrust slices within the imbricate zone of the CoastBelt Tlu-ust System to the west (Journeay and Friedman, 1993; Plate 1). The eastern boundary of the BridgeRiver terrane is the Yalakom/Hozameen fault system, a post-late Early Cretaceous dextral transpressionalstructure that has been offset by movement on the Fraser River-Straight Creek fault system.Stratigraphic relationships between the Bridge River terrane and adjacent terranes are ambiguous; theterrane is everywhere fault bounded (Plate 1). The contact between the Bridge River terrane and the Harrisonterrane to the west is an imbricate fault zone of the Late Cretaceous Coast Belt Thrust System (Journeay andFriedman, 1993; Fig. 3.2, Plate 1). The boundary between the Bridge River terrane and the Methow terrane tothe east are late Early Cretaceous to Tertiary transpressional structures associated with the YalakomHozameen and Fraser River fault systems. The terrane pinches out to the northwest against the Coast Plutonic50Complex and Yalakom fault, and pinches out to the southeast against the Hozameen and Ross Lake faults.Rusmore et al. (1988) suggest that the Middle to Late Jurassic Relay Mountain Group provides a stratigraphiclink between the Bridge River and Cadwallader terranes by late Middle Jurassic (Callovian) time. However,the Relay Mountain Group is everywhere fault-bounded, and the existence of a depositional contact between itand the Bridge River terrane has not been proven. The Lower Cretaceous Taylor Creek Group provides theoldest definitive tie between the Cadwallader and Bridge River terranes (Garver, 1992).D. CadwalladerThe Cadwallader terrane is exposed in the eastern Coast Belt, along the eastern side of the CoastMountains (Fig. 1.2). The base of the Cadwallader terrane is defined as the Cadwallader Group, whichconsists of Middle to Upper Triassic (Carnian to Norian) tholeiitic basalts of the Pioneer Formation andoverlying siltstone, sandstone, and limestone-bearing conglomerate of the Hurley Formation (Rusmore, 1987).The Hurley Formation is conformable with Upper Triassic (middle to upper Norian) conglomerate, sandstone,bioclastic sandstone, limestone, and siltstone of the Tyaughton Group (IJmhoefer, 1990). The TyaughtonGroup is disconformably overlain by Lower to Middle Jurassic (Hettangian-Bajocian) fine-grained clastics ofthe Last Creek Formation (Umhoefer, 1990; Poulton and Tipper, 1991), these in turn are unconformablyoverlain by sandstone, siltstone, and shale of the late Middle Jurassic to Lower Cretaceous (Callovian toBarremian(?)) Relay Mountain Group. Conglomerate and sandstone of Lower Cretaceous (Aptian to Albian)Taylor Creek Group unconfonnably overlies the Relay Mountain Group, and the Cadwallader terrane iscapped by the Upper Cretaceous (Cenomanian) Silverquick conglomerate and overlying Powell Creekvolcanics (Garver, 1989, 1992).The Cadwallader terrane occurs primarily in north to northwest-trending fault slices on the west sideof the Bridge River terrane, and as isolated pendants along the eastern margin of the Coast Plutonic Complex(Wheeler and McFeely, 1991). The Cadwallader terrane occupies both hanging wall and footwall positions in51the west-vergent Coast Belt Thrust System. North to northwest trending contractional and strike-slip faultsseparate it from adjacent terranes (Journeay and Friedman, 1993)The western margin of the Cadwallader terrane is dominantly an intrusive contact with late EarlyCretaceous plutons of the Coast Plutomc Complex (Armstrong and Parrish, 1990; Friedman and Armstrong,1994). The western margin of the Cadwallader terrane is also imbricated with Upper Cretaceous strata ofWrangellian and/or Harrison terrane affinity (Gambier Group) along the leading edge of the Coast Belt ThrustSystem (Journeay and Friedman, 1993). An apparent depositional contact between the Harrison andCadwallader terranes has been described by Journeay and Mahoney (1994; Appendix E).To the east, the Cadwallader terrane is everywhere in fault contact with the Bridge River terrane, andthere is no known stratigraphic connection prior to Late Cretaceous time (Garver, 1992). The Cadwalladerand Bridge River terranes are complexly imbricated along the western boundary of the Bridge River terrane byCretaceous to Tertiary compressional and transpressional structures (Schriazza et al., 1990; Monger andJourneay, 1994; Plate 1). Along the eastern margin of the Bridge River terrane, thin slivers of HurleyFormation are imbricated with fault slices of Bridge River Complex along the Yalakom fault system(Schriazza et al., 1990). Near bedding parallel faults of the Yalakom fault system separate the HurleyFonnation from Middle Jurassic and Lower Cretaceous strata of the Methow terrane to the east.E. MethowThe Methow terrane is exposed along the boundary of the morphogeologic Coast and Intermontanebelts, west of the Yalakom, Bridge and Fraser rivers, and east of the Thompson Plateau (Fig. 3.1). TheMethow terrane comprises Jurassic and Cretaceous clastic strata overlying Middle Tnassic oceanic basement(Fig. 3.2). The base of the Methow terrane is the Triassic Spider Peak Formation, a sequence of oceanicgreenstone, chert, limestone and argillite (Ray, 1986, 1990). The Spider Peak Formation is unconformablyoverlain by clastic strata of the Ladner Group (Ray, 1990). The Ladner Group consists of fine-grained52sandstone, siltstone, and carbonaceous shale of the Early Jurassic (Pliensbachian(?) to Toarcian) Boston BarFormation conformably overlain by volcaniclastic conglomerate, sandstone, siltstone, and minor volcanicrocks of the Lower to Middle Jurassic (Toarcian to Upper Bajocian) Dewdney Creek Formation (O’Brien,1986, 1987; Mahoney, 1993). The Ladner Group is disconformably overlain by pebble conglomerate,sandstone, and siltstone of the Upper Jurassic (Oxfordian to Kinuneridgian) Thunder Lake sequence (O’Brien,1986, 1987). Polymict, granitoid-bearing conglomerate and sandstone of the Lower Cretaceous(Hauterivian(?) to Albian) Jackass Mountain Group unconformably (disconformably(?)) overlie all older strata(Kleinspehn, 1982, 1985).The western boundary of the Methow terrane is the Yalakom/Hozameen fault system; the easternboundary is the Pasayten fault (Plate 1). The terrane is folded into northwest-trending asymmetric open folds,and locally cut by east-vergent thrust faults of moderate displacement (1-10 km; Coates, 1970, Monger, 1989).The Fraser fault system displaces the Methow terrane with approximately 100 km of dextral offset, fromBoston Bar to north of Lillooet (Fig. 3.2; Coleman and Parrish, 1992; Mahoney, 1993; Appendix E).The Yalakom-Hozameen fault system is a dextral transcurrent system (approximately 115 km offset;Riddel Ct al., 1993) that juxtaposes oceanic rocks of the Bridge River/Hozameen Complex (Bridge Riverterrane) with clastic strata of the Methow terrane. At the southern end of the Methow terrane, the Ross Lakefault zone separates Jurassic-Cretaceous clastic strata of the Methow terrane from amphibolite facies oceanic-type rocks of the Twisp Valley Schist, in the crystalline core of the North Cascades (Miller et al., 1993). Milleret al. (1993) correlate metamorphic rocks of the Twisp Valley Schist with oceanic rocks of the Bridge RiverHozameen Complex, and suggest that the Twisp Valley Schist is the product of thrust loading and plutonemplacement along a southern continuation of the Coast Belt Thrust System (Journeay and Friedman, 1993),or within slightly younger northeast- to east-directed thrust faults of the eastern Cascades foldbelt (McGroder,1988). This interpretation is consistent with the present map pattern that shows the Bridge River terranesensu strictu pinching out at the intersection of the Ross Lake and Hozameen faults (Fig. 3.2).53The eastern boundary of the Methow terrane is the Pasayten fault, which separates it from UpperJurassic to Lower Cretaceous plutomc rocks of the Eagle Complex and the Okanagan Batholith (Grieg, 1992;Hurlow and Nelson, 1993). The earliest documented movement on the Pasayten fault is Early Cretaceoussouthwest-side-down dip-slip motion, followed by moderate (10’s 1cm) sinistral displacement (Hurlow, 1993).However, the Pasayten fault is parallel to and spatially associated with the Eagle Shear Zone, a Middle to LateJurassic east-vergent contractional feature, and may be genetically related to this structure (Grieg, 1992;Hurlow, 1993).The Methow terrane is lenticular in plan view, narrowing both to the north and to the south (Fig. 1.2,3.2). To the south, the terrane is overlain by Miocene plateau basalts of the Columbia River Basalt Group(Stoffel et al., 1991). To the north, stratigraphic and structural relations within the Methow, Bridge River andCadwallader terrane are ambiguous. Riddell et al. (1993) assigned Jurassic-Cretaceous strata on the southwestside of the Yalakom fault to the Methow terrane, and correlated these strata near Konni Lake with those of theCamelsfoot Range to the south. This correlation suggests about 115 km dextral offset along the Yalakom fault(Fig. 3.2). In the Konni Lake area, rocks assigned to the Methow, Bridge River and Cadwallader terranes areeverywhere in fault contact, and stratigraphic relations are unclear. However, recognition of Methow stratawest of the Yalakom fault precludes the fault from being a terrane boundary between the Methow and theBridge River, as has been suggested to the south (Monger, 1985; Monger and Journeay, 1994).F. Cache CreekThe Cache Creek terrane is exposed northeast of Lillooet, east of the Fraser River, along thesoutheastern margin of the Chilcotin Plateau (Fig. 3.1). Tectonic slivers of the Cache Creek terrane areexposed in the Chilcotin River drainage (Read, 1993), but the majority of the terrane trends northward alongthe southeastern and eastern margin of the Chilcotin Plateau from approximately 50°30’N. This investigationis concerned with primarily the southern terminus of the -1500 km long terrane.54The Cache Creek terrane consists of chert, argillite, limestone, basalt and ultramafic rocks ofMississippian to Middle Jurassic age (Monger and McMillan, 1984). The terrane is divided into eastern,central, and western belts (Duffel and McTaggert, 1952; Monger and McMillan, 1989). The eastern beltcomprises greenstone and melange, consisting of blocks of limestone, tuff greenstone, bedded chert in avariably sheared matrix of carbonaceous argillite (Shannon, 1981). Microfauna from the eastern belt rangesfrom Middle Pennsylvanian to Late Triassic (Orchard, 1984; Cordey et al., 1986). The central belt consists ofmassive limestone of the Marble Canyon Formation with lesser amounts of argillite, tuff and chert (Shannon,1981; Monger and McMillan, 1984). Fusulinids and conodonts in the Marble Canyon Formation are Permian(Orchard, 1984); radiolaria in adjacent sedimentary rocks are Late Triassic (Cordey et al., 1986). The westernbelt consists of argillite, siliceous argillite, chert and volcaniclastic rocks, with subordinate limestone,conglomerate, and volcanic rocks (Trettin, 1980; Cordey et al., 1987). Volcanic lithic sandstone, siltstone,and siliceous volcanic rocks of the informally named “Pavilion beds” are included in the western belt (Trettin,1980; Monger and McMillan, 1989). Middle to Late Permian fusulinids and Early and Late Triassicconodonts have been documented from the western belt; Cordey et al. (1987) report Middle to Late Triassicradiolaria derived from chert interbeds, and Early to Middle Jurassic (Pliensbachian to Bajocian) radiolariaderived from siliceous argillite of the “Pavilion beds”.The Cache Creek terrane is in fault contact with both the Methow terrane and Quesnellia. Internalstructural complexities make relations between the three lithologic belts of the Cache Creek terrane itselfdifficult to assess (Trettin, 1980; Monger and McMillan, 1989). The Cache Creek terrane has long beenconsidered a subduction complex genetically associated with the Late Triassic Nicola arc (Monger et al., 1982,1991). However, the existence of Early to Middle Jurassic radiolaria in the “Pavilion beds” requiresedimentation until at least that time (Cordey et al., 1987).Numerous workers have speculated on correlations between the Cache Creek and Bridge Riverterranes (Potter, 1983; Haugerud, 1985; Cordey et al., 1987; van der Heyden, 1992). Arguments againstcorrelation of the two terranes are based primarily on the absence of both massive limestone and Tethyan55fauna in the Bridge River terrane (Potter, 1983; Haugerud, 1985; Cordey et al., 1987; Monger and McMillan,1989). Read and Cordey (1994) document an Early Jurassic(?) lithologic transition from pelagic oceanic rocksto clastic marine sedimentary rocks in the southern Cache Creek terrane, similar to that documented in theBridge River terrane (Journeay and Northcote, 1992; Mahoney and Journeay, 1993). They argue that thesouthern Cache Creek terrane may actually be a thrust nappe of Bridge River terrane, an interpretation thatrequires the present map pattern to be the result of transcurrent motion on the Yalakom-Hozameen and Fraser-Straight Creek fault systems (Wheeler and McFeely, 1992; Figs. 1.2, 3.2).G. QuesnelliaQuesnellia, the easternmost of the accreted terranes discussed in this investigation, comprises lowerMesozoic volcanic assemblages overlying Upper Paleozoic oceanic and arc sequences (Monger, 1989). Theareally restricted Paleozoic basement of Quesnellia comprises argillite, sandstone, chert, limestone, and arcvolcanic rocks of the Devonian to Permian Harper Ranch Group. The Paleozoic rocks are unconformablyoverlain by voluminous volcanic, volcaniclastic and sedimentary rocks of the Upper Triassic to Lower JurassicNicola Group that characterize Quesnellia (Mortimer, 1987). The Nicola Group is unconformably overlain byshale, siltstone, and sandstone of the Lower to Middle Jurassic (Sinemurian - Callovian) Ashcroft Formation,which overlies both Nicola Group volcanic rocks and comagmatic plutonic rocks (McMillan, 1974; Mongerand McMillan, 1989). Along the western margin of Quesnellia upper Lower Cretaceous volcanic rocks of theSpences Bridge Group occupy a >200 km long structural depression, and unconformably overlie the NicolaGroup and associated plutonic rocks, the Permian to Lower Mesozoic Mt. Lytton Complex, and the CacheCreek terrane. Hurlow and Nelson (1993) suggest comagmatic rocks to the Spences Bridge Group extendsouth into Washington, where they comprise the Cretaceous Okanagan Batholith.In the southern Canadian Cordillera, the western boundary of Quesnellia is the Pasayten fault, whichseparates Jurassic-Cretaceous rocks of the Methow terrane from Mesozoic intrusive complexes and Tnassicand Cretaceous arc volcarncs of western Quesnellia (Wheeler and McFeely, 1991; Stoffel et a!., 1991). North56of Lillooet, the western margin of Quesnellia is a fault contact with the Cache Creek terrane (Monger, 1989).The eastern boundary of Quesnellia is a series of extensional faults, such as the Okanagan fault, associatedwith Eocene uplift of the Omineca Crystalline Belt (Parrish et al., 1985). North of approximately 50.5°N, theeastern margin of Quesnellia stratigraphically and structurally overlies the pericratonic Kootenay terrane(Monger et al., 1991).Quesnellia is interpreted to have been obducted onto the North American continental margin in lateEarly Jurassic time (—185 Ma; Monger, 1989; Monger et aL, 1991; Ghosh and Armstrong, 1994). The easternmargin of Quesnellia is interpreted to be an Early Jurassic structural contact with pencratonic terranes to theeast, overprinted by Jurassic to Tertiaiy intrusions, and modified by Eocene extensional faulting (Armstrongand Parrish, 1990; Ghosh and Armstrong, 1991; Monger et al., 1991). Quesnellia may therefore be viewed asthe Middle Jurassic western edge of North America.The earliest documented interaction between Quesnellia and the Methow terrane to the west was thedeposition of the Lower Cretaceous Jackass Mountain Group on the Methow terrane, which received detritusfrom the Upper Jurassic-Lower Cretaceous Eagle Complex on the western edge of Quesnellia (Kleinsphen,1982). The Eagle Complex displays evidence of Middle to Late Jurassic ductile deformation, but therelationship between this deformation and the Methow terrane is uncertain (Grieg, 1992).3.2 STRUCTURAL SETTINGThe present map pattern of the southern Canadian Cordillera is derived from Cretaceous to Tertiarycompressional, transcurrent, and extensional structures generated during oblique convergence along the NorthAmerican margin. Older (pre-Cretaceous) structures are strongly overprinted by younger structures orobliterated by plutons.57The oldest clearly identifiable structures in the southern Canadian Cordillera are early Mesozoic (LateTriassic-Early Jurassic) in age, and are associated with deformation in the Cache Creek terrane andQuesnellia. Monger (1985) reports Late Triassic to Early Jurassic deformation and metamorphism in the MtLytton Complex of western Quesnellia, and McMilIan (1976) describes structures associated withemplacement of the Late Triassic-Early Jurassic Guichon Batholith. Numerous workers have described Earlyto Midcile Jurassic deformation along the eastern margin of Quesnellia (Kiepalci, 1985; Brown et al., 1986,Murphy et al., 1992). Late Triassic to Early Jurassic structures in the Cache Creek terrane, Quesnellia, andterranes to the east are interpreted to be associated with the obduction of Quesnellia onto North America(Monger et al., 1982).Middle and Late Jurassic structures are documented in most terranes in the region. Friedman et al.(1990) and Monger (1991a) document Middle to Late Jurassic (post-185 Ma, pre-155 Ma) deformation of theBowen Island Group in Wrangellia. Monger and McNichol (1994) document a post-Late Triassic deformedbelt cut by a late Middle Jurassic (ca. 164 Ma) pluton on the eastern edge of Wrangellia. Rusmore et al. (1988)argue that folds and thrusts in Upper Triassic and Lower Jurassic strata of the Cadwallader terrane are notfound in overlying Upper Jurassic strata, indicating a Middle Jurassic defonnational episode. Middle and LateJurassic structures have not been identified in Harrison, Bridge River, or Methow terranes. The Eagle shearzone on the west side of Quesnellia records ductile contractional deformation in the Middle to Late Jurassic(Grieg, 1992). Travers (1982) documents east-directed thrust faulting of Cache Creek terrane over Quesnelliain the Middle to Late Jurassic. This deformational event is supported by evidence of penetrative structuraldeformation and metamorphism in the Cache Creek terrane (Mortimer et al., 1990) and in the Mt. LyttonComplex of western Quesnellia (Grieg, 1992; Fig. 3.2, Plate 1).Early Cretaceous deformation is ciyptic in the southern Canadian Cordillera, and is primarilymanifested by unconformities and thick synorogenic conglomerate sequences on all terranes (Kleinsphen,1982; Arthur, 1987, Lynch, 1992; Garver, 1992; Mahoney and Journeay, 1993). Monger (1993) proposesEarly Cretaceous extensional deformation in the western Coast Belt to account for exhumed Late Jurassic58plutons, oriented dike complexes, and sedimentologic and volcanic patterns in Lower Cretaceous strata.Definitive Early Cretaceous structures have not been identified in the Harrison, Bridge River, Cadwallader, orMethow terranes, although the presence of Lower Cretaceous grarntoid-bearing conglomerates suggestssubstantial (>5 km) coeval uplift. Early Cretaceous structures in Quesnellia and the Cache Creek terrane areunknown.Late Early Cretaceous (< 112 Ma; early Albian) to Late Cretaceous time heralded the beginning of amajor deformational episode on all terranes. The most regionally significant structural feature formed duringthis period is the west-vergent Coast Belt Thrust System (Journeay and Friedman, 1993), which iskinematically linked to the Northwest Cascade System to the south (Brown, 1987; Brandon et al., 1988;McGroder, 1991; Journeay and Friedman, 1993). West-vergent deformation was preceded by local post-middle Albian south- to southeast-directed folds and thrust faults (Lynch, 1991, 1992). Shortening in theCoast Belt Thrust System consisted of southwest-directed thin-skinned thrust faulting followed by out-of-sequence thrusting and folding; deformation is bracketed by syn- and post-kinematic plutons at 9 1-97 Ma.Deformation in the Coast Belt Thrust System imbricates units of Harrison, Bridge River, Cadwallader, andMethow affinity (Journeay and Friedman, 1993). Early Late Cretaceous (9 1-94 Ma) deformation in thewestern Coast Belt consists of discrete, west- to southwest-vergent contractional shear zones developedprimarily in plutomc rocks (Monger, 1993; Monger and Journeay, 1994). Journeay and Friedman (1993)suggest that plutonic suites of the western Coast Belt acted as a rigid backstop during Late Cretaceousshortening. To the north, Rusmore and Woodsworth (1991) describe a system of east-vergent backthnistsantithetic to the main thrust system that was active from 87-84 Ma. To the south, west-directed thrust faultingin the Northwest Cascade System and metamorphism in the core of the Cascades is loosely constrained to bebetween 130-84 Ma, with most isotopic dates between 100-84 Ma (Brown, 1987; Brandon et al., 1988;McGroder, 1991). Antithetic, east-vergent thrust faults in the Methow terrane are coeval with thisdeformation (McGroder, 1991).59Ductile, sinistral displacement along the Pasayten fault between the Methow terrane and Quesnellia isconstrained to be ca. 109-95 Ma, and is interpreted to be coeval with west-vergent contractional faulting alongits southern extent (Greig, 1992; Hurlow, 1993). On Quesnellia, the northwest-trending Nicoamen synclinedeveloped coeval to deposition of the upper Lower Cretaceous Spences Bridge Group (Monger et al., 1991).Additional regionally significant early Late and Late Cretaceous structures have not been identified east of thePasayten fault.Transcurrent displacement overlapped with, and continued after, cessation of contractionaldefonnation throughout the region. Transcurrent, primarily dextral, movement on a complex system of north-to northwest-trending strike-slip faults in the region began in the early Late Cretaceous and continued into theTertiary (Schriazza et al., 1990; Journeay et al., 1992; Parrish and Monger, 1992). The aggregatedisplacement on these fault systems is uncertain. Dextral transcurrent motion along the Yalakom fault systemis interpreted to be kinematically linked with Eocene (5547 Ma) extensional displacement (Coleman andParrish, 1992). To the east, major extensional faulting in the southern Omineca Belt, along the eastern borderof Quesnellia, occurred between 58 and 45 Ma and resulted in 60-80% extension (Parrish and Carr, 1986;Parrishetal., 1988).Dextral transcurrent displacement of approximately 100 km occurred along the Fraser fault systembetween 46.5 and 34 Ma, offsetting both strata of the Bridge River, Methow and Cache Creek terranes, andhigh-grade metamorphic rocks of the Southern Coast Belt and Northwest Cascades (Coleman and Parrish,1991; Mahoney, 1993; Monger and Journeay, 1994).3.3 PLUTOMC SETTINGPlutonic rocks constitute a major proportion of the southern Canadian Cordillera; west of the FraserRiver, plutons underlie between 60-80% of the region. The vast majority of plutons in the region are Jurassicto Tertiary in age. Plutonic rocks in the region may be broadly divided into two tracts: 1) an eastern tract,60east of the Pasayten fault, coincident with the Intermontane belt, including intrusions into Cache Creek,Quesnellia, and terranes to the east, and; 2) a western tract, west of the Pasayten fault, coincident with theCoast and Insular belts, including intrusions into Bridge River, Cadwallader, Harrison and Wrangelliaterranes. The Methow terrane is largely amagmatic. The two tracts display differences in pluton age,distribution and geochemical and isotopic composition, although there is a strong degree of overlap betweenthe two (Woodsworth et al., 1991)The eastern tract is generally older, more randomly distributed, cuts across structural grain, and ismore silicic than intrusions in the western tract (Woodsworth et al., 1991). The eastern tract generallydisplays more evolved strontium and neodymium isotopic values than the western tract (Ghosh, 1994;Friedman and Cui, 1994). The most voluminous plutonic suites in the eastern tract are Late TriassicfEarlyJurassic and Middle to Late Jurassic, with areally less significant suites in the late Early to Late Cretaceousand Tertiary (Annstrong, 1988; Woodsworth et al., 1991; Hurlow and Nelson, 1993). Different age plutonicsuites overlap each other without a consistent pattern (Wheeler and McFeely, 1991). Although early LateCretaceous magmatic activity occurred throughout the eastern tract, the majority of this activity occurred in a>300 km north- to northwest-trending belt along the western margin of Quesnellia referred to as theOkanagan-Spences Bridge arc (Hurlow and Nelson, 1993). Tertiary intrusions are particularly widespreadeast of Quesnellia.The Coast Plutomc Complex forms the vast majority of the western tract. Plutomc rocks are roughlyaligned in northwest-trending belts of similar age, are intermediate in composition, and have juvenile isotopicvalues (Woodsworth et al., 1991; Friedman and Armstrong, 1994; Friedman and Cm, 1994; Cui and Russell,1994). The oldest plutons are the Lower Jurassic Island intrusions, on the western side of Wrangellia, whichare interpreted to be the intrusive equivalent of arc volcanics of the Bonanza Group (Andrew and Godwin,1989a). Middle to Late Jurassic (167-145 Ma) plutons comprise the early Coast Plutomc Complex suite(ancestral Coast Belt of van der Heyden, 1992; Friedman and Armstrong, 1994). Plutons of this age extendacross the western Coast Belt, and are contained in two parallel belts, separated by younger intrusions61(Friedman and Armstrong, 1994). Early Cretaceous (145-112 Ma) plutons are concentrated along the westernmargin of the Coast Belt (Fig. 3.2, Plate 1). Mid-Cretaceous (112-90 Ma) plutons are coeval with deformationin the Coast Belt Thrust System, and intrude all terranes west of the Pasayten fault. Late Cretaceous intrusionspost-date the main phase of contractional deformation in the Coast Belt, and appear confined to the hangingwall of the Coast Belt Thrust System (Journeay and Friedman, 1993). These plutons are exposed east of themajority of older plutons, and represent a significant (85-115 kin) eastward shift of magmatic activity.Tertiary intrusions occur as isolated plutons scattered throughout the western tract, but form an importantcomponent of the North Cascades region (Haugerud et al., 1991).62CHAPTER 4EVOLUTION OF A MIDDLE JURASSIC VOLCANIC ARC: STRATIGRAPHIC,ISOTOPIC AND GEOCIIEMICAL CI[ARACTERISTICS OF THE HARRISONLAKE FORMATION, SOUTHWESTERN BRITISH COLUMBIA634. EVOLUTION OF A MIDDLE JURASSIC VOLCANIC ARC: STRATIGRAPUIC, ISOTOPIC ANDGEOCHEMICAL CHARACTERISTICS OF THE HARRISON LAKE FORMATION,SOUTHWESTERN BRITISH COLUMBIA4.1 INTRODUCTIONEarly to Middle Jurassic arc sequences form an important component of many terranes in theCanadian Cordillera. The genetic relationships among these coeval sequences and the originalpaleogeographic configuration of the terranes they comprise are largely unknown. Documentation of thegeologic and geochemical evolution of these Middle Jurassic arc sequences is required prior to any attempt atcomparison between coeval sequences. The Harrison Lake Formation of the Harrison terrane is the mostextensive Early to Middle Jurassic arc sequence preserved in the Coast Plutomc Complex in the southernCanadian Cordillera (Fig. 4.1). Adjacent terranes contain coeval volcano-sedimentaiy assemblages ofuncertain affinity. Documentation of the geologic and geochemical characteristics of the Harrison LakeFormation thus provides a framework within which to evaluate potentially correlative strata on adjacentterranes. This chapter describes the stratigraphy, volcanic geochemistry, isotopic characteristics and U-Pbgeochronology of the Harrison Lake Formation, discusses the geologic evolution of the formation, and brieflyoutlines intraformational mineral deposits. This study represents the first detailed geologic and geochemicaldescription of the Harrison Lake Formation.4.2 GEOLOGIC SETTINGThe Harrison terrane, exposed west of Harrison Lake, contains the most intact and well preservedMesozoic stratigraphic section in the southwestern Canadian Cordillera (Fig. 4.1; Monger, 1970). The terraneconsists of Middle Triassic oceanic rocks of the Camp Cove Formation unconfonnably overlain by arc volcanicrocks of the Early to Middle Jurassic Harrison Lake Formation, that in turn are unconformably overlain byMiddle to Upper Jurassic clastic rocks of the Mysterious Creek and Billhook Creek formations (Monger, 1985;64Harrison LakeFormationHarrison TerraneCoast plutostc Complexundivided sodasedisoentiotihe eastern Coast BeltFigure 4.1- a) morphogeologic belts of the Canadian Cordillera, with study area outlined. Modified fromMonger and Hutchinson (1971); b) generalized terrane map of the southern Canadian Cordillera,illustrating terranes mentioned in text. Modified from Wheeler and McFeely (1991); c) schematicstratigraphic column of the Harrison Lake terrane. Modified from Arthur (1987).WrangelliaHarrisonCadwaliaderBridge RiverMethowQuemselliaCods. CreekSflkiabKootessayCassiarterranes of Northweit CascadesAAAAAA A AA AAA A AA AA AAAA A A A AAAfAAAAAAAAAAA\\ AAAAAAAAAAA\ Af’\AAA .‘Brokonback 14111 FonssstlouPeninsula FormationBIUhOOIL Crest Formatio.Mysterious Creek FormulaeCamp Cove Formation65Arthur, 1986, 1987; Arthur et aL, 1993). Lower Cretaceous polymict conglomerate of the PeninsulaFormation and volcanic strata of the Brokenback Hill Formations unconformably cap the sequence.The Harrison terrane is intruded by Middle Jurassic and late Early Cretaceous plutons of the CoastPlutonic Complex to the west (Friedman and Armstrong, 1994; Fig. 4.1). The eastern boundary of the terraneis the Harrison Lake shear zone, a dextral transcurrent fault that separates subgreenschist facies rocks of theHarrison terrane from amphibolite facies metamorphic rocks within the Coast Belt Thrust System to the east(Journeay and Csontos, 1989; Journeay and Friedman, 1993). Rocks within the imbricate zone at the leadingedge of the west-vergent Coast Belt Thrust System have been correlated with the Harrison terrane (Journeayand Friedman, 1993; Journeay and Mahoney, 1994), suggesting that rocks of the Harrison terrane extendfarther east and north than previously recognized (Monger, 1985; Arthur et al., 1993)The southern boundary of the terrane is the steeply dipping Vedder fault, which separates LatePaleozoic limestone and Jurassic-Cretaceous clastic rocks of the Chilliwack terrane from the Harrison terrane.The structural and stratigraphic relationship between the Harrison and Chilliwack terranes is enigmatic.Paleozoic limestone clasts found in the basal conglomerate of the Harrison Lake Formation are inferred to bederived from the Chilliwack terrane, thus suggesting an Early Jurassic stratigraphic linkage between the two(Arthur, 1987; Monger and Journeay, 1992; Arthur et al., 1993). Structural relations in the Cascade range tothe south suggest equivalents of the Chilliwack terrane structurally overlie rocks that have been correlated withthe Harrison terrane (Misch, 1966; Tabor, 1994).The Harrison Lake Formation comprises the southern two-thirds of the Harrison terrane. It isprimarily exposed northeast of the Chehalis River, north of the Fraser River, south of Mystery Creek, and,with the exception of exposures on Echo Island, west of Harrison Lake (Figs. 4.1, 4.2 - in pocket). Strata ofthe Harrison Lake Formation are exposed in a poorly developed west-northwest trending, shallowly westerlyplunging open anticline. The base of the formation is exposed in the core of the anticline approximately 2 kmsouth of Camp Cove (Fig. 4.2). The northern limb of the anticline is truncated in part by a steeply dipping66northwest-trending structure near Camp Cove that juxtaposes the upper members of the formation withTriassic basement of the Camp Cove Formation (Fig. 4.2). The formation is cut by a myriad of smallernorthwest and northeast-trending faults of modest (tens to hundreds of metres) offset.4.3 PREVIOUS WORKCrickmay (1925) described the volcanic rocks along the west side of Harrison Lake, subdividing theminto the Harrison Lake, Echo Island, and Mysterious Creek formations, and established a Middle Jurassicbiostratigraphic age for the main volcanic member. Thompson (1972) and Pearson (1973) mapped thesouthern portion of the outcrop belt to constrain the regional setting of Kuroko-style stratiform mineralizationin the Seneca deposit (Fig. 4.2). Arthur (1986, 1987) and Arthur et al. (1993) described the formation,subdivided the Harrison Lake Formation into four formal members (Celia Cove, Francis Lake, Weaver Lake,and Echo Island members) and established a late Early Jurassic to Middle Jurassic (Middle Toarcian to midAalenian) biostratigraphic age for the lower portion of the formation. McKinley et al. (1994) described thelocal stratigraphy and mineralization of the Seneca deposit in the southeastern portion of the study area.Arthur (1987) mapped the west side of Harrison Lake at a scale of 1:50,000 during a biostratigraphicstudy of Jurassic and Cretaceous strata. Monger (1989) mapped the west side of Harrison Lake at a scale of1:250,000. Thompson (1972) and Pearson (1973) mapped the area south of Mt. Klaudt at a scale ofapproximately 1:35,000, concentrating on economic mineralization. In this investigation, the outcrop area ofthe Harrison Lake Formation was mapped for the first time in its entirety at a scale of 1:50,000 (Fig. 4.2).4.4 STRATIGRAPHYThe four-fold subdivision of the Harrison Lake Formation established by Arthur et al. (1993) isadapted herein (Fig. 4.3). The following descriptions are a significant modification and expansion of the67definitions of Arthur et a!. (1993), and are based on extensive regional mapping, examination of lateral andvertical facies changes, drill hole data, and petrographic information.Strata of the Harrison Lake Formation are predominantly volcanic lava flows and volcaniclasticsedimentaiy rocks. In this manuscript, volcaniclastic sedimentaiy rocks are distinguished according to theirmode of deposition, and include: 1) pyroclastic sediments, which are direct products of pyroclastic eruptiveprocesses, and display diagnostic features such as fianune and glass shards; 2) resedimentedpyroclasticdebris, which consists of dethtus initially formed by pyroclastic processes that has been redistributed prior tolithification by normal fluvial and marine processes; and 3) epiclastic sediments, which are clastic sedimentsformed by the normal weathering and erosion of pre-existing volcanic deposits (Fisher and Schminke, 1984;Cas and Wright, 1987: McPhie et al., 1994).A. Celia Cove MemberThe Celia Cove Member is a basal conglomerate unit unconformably overlying greenstone, chert,siltstone and sandstone of the Triassic Camp Cove Formation. The Celia Cove Member is only exposed in thecore of the Camp Cove anticline (Fig. 4.2). The basal contact appears disconfonnable adjacent to massivegreenstone, but an angular unconformity between the two is indicated by: 1)1-2 m amplitude tight folds inchert and siltstone beds of the Camp Cove Formation along the lake shore south of Camp Cove that are absentin overlying rocks; 2) macroscopic folds suggested by structural orientation of bedded sediments in the CampCove Formation and not documented in immediately overlying strata; and 3) angular discordance betweennortheast dipping argillite of the Camp Cove Formation and west dipping conglomerate of the Celia CoveMember north of Francis Lake (Fig. 4.2).The Celia Cove Member consists of volcanic and chert pebble to cobble conglomerate and medium tocoarse-grained lithic sandstone. The base of the member is characterized by 1-2 m of granule to cobble-sizedangular to subangular clasts of light grey to dark blue chert and lesser greenstone floating in a coarse lithic68—.t-40CD CDCDCDCD CD——.C..(DOOCDCDo—,00..0CDCDN po ..l—.ncno CD.CDzCDarenite matrix. Clasts become more well rounded, the sandstone content increases, and the conglomeratebecomes more polymictic upsection, where it contains clasts of andesite porphyry, dacite porphyry, chert,volcanic lithic arenite, and rare well-rounded bioclastic limestone. Chert and limestone clast content decreasesdramatically a few metres above the basal contact. Clast size reaches a maximum (cobble to boulder size)about 10 m above the base, then becomes progressively finer grained upsection. The conglomerate is thickbedded (1-2 m), locally clast supported, and intercalated with medium bedded matrix supported pebble togranule conglomerate and coarse-grained lithic sandstone. Contacts between beds are gradational;conglomerate beds become gradationally finer grained and thinner bedded upward, whereas the overlyingsandstone and granule conglomerate beds coarsen and thicken as they grade into the next overlyingconglomerate. Sandstone intercalated with the conglomerate is medium to coarse grained, moderately sorted,and is composed of sausseritized lithic volcanic fragments, plagioclase, and minor chert, quartz and sanidine.Thickness of the Celia Cove member varies from 10-60 m (Fig. 4.3).The contact between the Celia Cove Member and the overlying Francis Lake Member is gradational,and is placed at the stratigraphically highest granule to pebble conglomerate bed. Conglomerate isgradationally overlain by thin to medium bedded, medium to coarse grained volcanic lithic wacke and siltstoneof the lower Francis Lake Member. The transition from crudely stratified cobble conglomerates of the CeliaCove Member to sandstone and siltstone of the Francis Lake Member represents an overall fining upwardstratigraphic succession.B. Francis Lake MemberThe Francis Lake Member gradationally overlies the Celia Cove Member. The unit weathersrecessively, and is best exposed along Harrison Lake 1.5 km north of Celia Cove, and in roadcuts on theHarrison West Forest Service Road east of Francis Lake. It is conformable with the Celia Cove Member, andtherefore mimics its arcuate outcrop pattern in the core of the Camp Cove anticline (Fig. 4.2). The Francis70Lake Member is estimated to be approximately 350-400 m thick, based on outcrop pattern and structuralattitudes west of Celia Cove.The Francis Lake Member is primarily thin bedded and fine grained, and comprises siliceousmudstone, calcareous siltstone, volcanic lithic wacke, ciystal vitric tuff and lapilli tuff Strata within themember initially fine upward from the underlying conglomerate, and then coarsen upward, incorporatingincreasingly larger proportions of crystal vitric tuff, lapilli tuff, and coarse grained tuffaceous feldspathic lithicwacke below the contact with the overlying volcanic Weaver Lake Member.In the lower portion of the Francis Lake member, thin to medium bedded medium to coarse grainedfeldspathic lithic wacke above the Celia Cove Member grades upward over a 15-30 m interval into thinbedded, parallel laminated siltstone intercalated with minor thin bedded (3-5 cm) normally graded finegrained feldspathic wackes. Sedimentary structures include graded bedding, basal scour features, and parallellaminae. The fining upward sequence initiated in the Celia Cove Member is capped by a 15-20 m thick,ammonite-bearing, thin bedded shale and siltstone interval. The upper portion of the shale interval isinterbedded with crystal tuff (Fig. 4.4). Grain size and bedding thickness increase upsection, where themember is dominated by thin to medium bedded medium to coarse grained tuffaceous lithic wackes andmassive-appearing reddish and greenish black siliceous siltstone interbedded with thin to medium beddedcrystal vitric tuffs and lapilli tuffs. Sedimentary structures are limited to parallel laminations, normally gradedbedding, and minor scour features. Ammonite and pelecypod fragments are locally evident. Matrix-supportedmonomict vitric tuff breccia, consisting of a single layer of pebble to cobble sized angular clasts of light-colored siliceous vitric tuff floating in a dark grey siliceous siltstone locally forms a distinctive component ofmassive siltstone and mudstone intervals. The upper portion of the member is dominated by medium to thickbedded, medium to coarse grained tuffaceous lithic feldspathic wacke interbedded with thin crystal tuffPyroclastic and epiclastic rocks of the upper Francis Lake Member are locally in sharp contact withoverlying augite andesite porphyry of the basal Weaver Lake Member, but the presence of abundant71volcaniclastic rocks interbedded with massive flows in the lower portion of the Weaver Lake Member suggestthe contact is gradational and conformable.C. Weaver Lake MemberThe Weaver Lake Member is the most areally extensive and volumetrically significant component ofthe Harrison Lake Formation. It is exposed over an area of over 150 km2 between Weaver Lake and Mt.McRae (Fig. 4.2), and forms the prominent topographic highs between the Chehalis River and Harrison Lake.Laterally discontinuous units and the poor exposure of sedimentary interbeds makes structural interpretationsdifficult; the unit is cut by numerous small faults and mesoscale (1-10 m amplitude) open folds are locallyevident.The Weaver Lake Member comprises porphyritic andesite, dacite and locally rhyolite lava flows,associated flow breccia, hyaloclastite, volcanic breccia, luff breccia, lapilli luff, and minor intercalatedepiclastic conglomerate, sandstone and siltstone. The member is dominated by lava flows, breccia, and luffbreccia that form prominent, resistant exposures. Individual units are lenticular and laterally discontinuous,and amalgamate into overlapping and interdigitating sequences with no stratigraphic continuity. Flows aregenerally massive, but locally display trachytic textures, amygdules, and brecciated flow tops. There is anoverall transition from andesitic flows near the base of the member into flows of predominantly daciticcomposition, followed by the rhyolitic units that characterize the upper portions of the member. The majorityof the flows are dacitic in composition, characterized by plagioclase and lesser hornblende and quartzphenocrysts. Volcanic breccias and tuffbreccias are most commonly matrix supported, vary from massive tocrudely bedded (0.5-10 m thick), appear lensoidal, and locally contain porphyritic clasts with distinctalteration rinds. North of the outflow of Harrison Lake, a clast supported volcanic breccia containing large (>30 cm) angular porphyritic clasts is interpreted as a vent-proximal agglomerate. V72It is commonly difficult to distinguish between volcanic flows and synvolcanic dikes and sills in theWeaver Lake Member. This investigation suggests that there are a much higher percentage of synvolcanicdikes and sills within the Weaver Lake Member than earlier workers (Thompson, 1972; Pearson, 1973;Arthur, 1985, 1987; Arthur et al., 1993). Dikes are recognized by the presence of chilled margins, sharpcontacts, lack of flow brecciation, cross-cuffing relations with adjacent flows and sediments, blocky weatheringcharacter, relatively fresh appearance, and ubiquitous columnar jointing. Columnar jointing described byArthur et al. (1993) as a characteristic flow feature actually occurs in cross-cutting dikes similar incomposition to overlying volcanic flows (Fig. 4.5). There is very little textural, mineralogical, or bulkcompositional difference between dikes and flows, and genetic classification must be based on field criteria.A spectacular rhyolite dome complex forms the uppermost Weaver Lake Member on the east side ofthe study area. The complex is well exposed on the north and west side of Echo Island, where it forms cliffsover 160 m high. The rhyolite is light grey to pinkish red, ranges from aphanitic to porphyritic, and has aphenociyst assemblage variably dominated by plagioclase, chloritized hornblende, or quartz. The unit islocally massive, but commonly displays well-developed flow foliation and flow folds that lead to a beddedappearance in outcrop. Parallelism between flow foliation and overlying sediments, variation in phenoczystassemblages, lack of quench fraginentation/brecciation at the upper or lower surface, and the absence of aradial pattern in dip direction within the overlying sediments suggests the rhyolite may have been emplaced asa series of shallow hypabyssal intrusions rather than as a cryptodome or flow sequence.Sedimentary sequences intercalated with primary volcanic rocks of the Weaver Lake Member includethin to thick bedded lapilli tuff, tuff tuffaceous sandstone, siltstone, and epiclastic conglomerate. Thin tomedium bedded immature tuffaceous lithic feldspathic wacke, tuffaceous siltstone and mudstone dominatesedimentary interbeds. Sedimentary structures include parallel and convolute laminae, trough cross laminae,flame structures, scour features, graded bedding, slump features, and small channels (approximately 1 mwide). Woody debris is locally abundant, and is found on bedding planes at the base of mass sediment gravityflow units. Sedimentary intervals tend to weather recessively, and locally may comprise 40-60% of the unit.73;I - --aa — a R—‘:.- •;pI - .— -. - I::4I i? —,i.3i. £Figure 4.5 - Photograph of dacite dike with well-developed columnar jointing cross-cutting altered dacite lavaflow of the Weaver Lake Member.Figure 4.4 - Photonucrograph of trachytic crystal tuff of the Francis Lake Member.74The thickness of the Weaver Lake Member is difficult to ascertain because upper and lower contactsare not exposed within the same structural block, outcrop is discontinuous, marker beds are rare, and a paucityof younging indicators restrict assessment of structural and stratigraphic continuity. Crickmay (1925)measured 2800 m of volcanic strata north of Camp Cove in a section cut by faults and containing open folds ofkilometre wavelength. Thompson (1972) estimated a minimum thickness based on topographic relief in thesouthern end of the outcrop area to be --1400 m. In this investigation, the approximate thickness of theWeaver Lake Member, based on simplified fold geometry and structural cross sections, was determined to beabout 2600 m. This value should be taken as a crude estimate at best.The contact between the Weaver Lake Member and the overlying Echo Island Member is placed atthe top of the uppermost lava flow or volcanic breccia in the Weaver Lake Member (Fig. 4.3). The nature ofthe contact varies across the study area due to the lateral variability of primary volcanic facies in the WeaverLake Member. Thin to medium bedded sedimentary rocks of the Echo Island Member overlie a rhyolite domecomplex on the south end of Echo Island, dacitic lava flows and breccia on the north slope of Mt. McRae, anddacitic breccia east of Moms Creek, north of the Harrison River (Fig. 4.2). The contact is easily located by aprominent break in topography and by the appearance of thick sequences of well-bedded volcarnclasticsedimentaiy rocks.D. Echo Island MemberThe Echo Island Member is the uppermost subdivision of the Harrison Lake Formation. The memberis well exposed along the south shore of Echo Island, along both sides of the Harrison River east of MorrisCreek, and on the north and east flanks of Mt. McRae (Fig. 4.2). The Echo Island Member conformablyoverlies the Weaver Lake Member; the contact is well exposed on Echo Island and the north flank of Mt.McRae (Fig. 4.2). The top of the member is an angular unconfonrnty with the overlying Mysterious CreekFormation. The member is well-bedded, and generally forms gently dipping homoclines. However, metre-75scale open folds are locally evident, and overturned bedding is mapped on the north flank of Mt. McRae. Themember is locally cut by dacitic dikes interpreted to be comagmatic with flows in the Weaver Lake Member.The Echo Island Member comprises, in decreasing order of abundance, volcaniclastic sandstone,siltstone, mudstone, granule to cobble conglomerate, lapilli tuff crystal to vitric tufl and minor tuffbreccia.Resedimented pyroclastic debris predominates, with subordinate epiclastic and pyroclastic deposits. Themember is dominantly thin- to medium-bedded, medium- to coarse-grained lithic feldspathic wacke andtuffaceous siltstone. Thick-to very thickly-bedded lithic feldspathic wacke, granule conglomerate, and lapillituff are also present, and form prominent outcrops. Massive to crudely laminated sandstone beds locallyamalgamate into units> 7 m thick. Cyclic sedimentation is evident in abundant fining and thinning upwardand coarsening and thickening upward stratigraphic sequences (10-35 m). Sedimentary structures includeparallel and convolute laminae, normal and inverse graded bedding, trough cross-bedding, channels, basalscour features, rip up clasts, load casts, soft sediment deformation, and rare complete Bouma (ABCDE)sequences. Thin bedded structureless siltstone and mudstone beds are commonly bioturbated.Sedimentary rocks of the Echo Island Member are dominantly thin to medium bedded volcanicsandstone, siltstone, mudstone and lesser crystal tuff This pattern is interrupted by thick-bedded to massive(2-15 m) lapilli tuff, tuffbreccia, granule to pebble volcanic breccia or tuffaceous sandstone. These thick unitsare normally graded, although they may have a basal inversely graded segment (< 1 m), and are characterizedby crude banding, bedding parallel clast alignment and relict fiamme. Normal grading is defined by adecrease in both grain size and the density and size of fiamme. In thin section, these units contain subangularto subrounded clasts of crystal luff, vitric tufl and lesser andesite and dacite porphyry floating in an alteredmatrix of clay and plagioclase microlites. These thick bedded intervals commonly overlie thin bedded finesandstone, siltstone and mudstone, and are overlain by thin- to medium-bedded fine to coarse grained lithicfeldspathic wacke containing abundant sedimentary structures. On the south end of Echo Island, clastsupported channelized and unchannelized pebble to cobble volcanic conglomerate occurs above one of the76massive intervals; the conglomerate fines and thins upward into thin to medium bedded parallel laminatedvolcaniclastic sandstone and siltstone.The total thickness of the Echo Island Member is uncertain. The most complete section of themember occurs along the Harrison River, east of Morris Creek, where the thickness is estimated to be 600-650m (Fig. 4.3). Arthur et al. (1993) suggest that the member thickens to the north, to about 1000 m north of Mt.McRae. However, this section is complicated by mesoscale folding, including overturned bedding, so thisestimate must be treated with caution.The top of the Echo Island Member is an unconformity with the overlying Mysterious CreekFonnation. The contact is disconformable in exposures along Harrison River (Fig. 4.2). The contact is notexposed to the north, but on the north flank of Mt. McRae steeply south-dipping overturned beds andmesoscale folds of the Echo Island Member appear to be overlain by gently north-dipping beds of theCallovian Mysterious Creek Formation. This relationship suggests an angular unconformity separates the two.4.5 AGE CONSTRAINTSAge constraints in the Harrison Lake Formation are provided by ammonite biostratigraphy and U-Pbzircon geochronology.A. Biostratigraphic DataFossils are rare in the Harrison Lake Formation. Crickmay (1925) reported probable Middle Jurassicbrachiopods, pelecypods, and possible corals from sandstone of the Francis Lake or Weaver Lake Members.H. Frebold (in Monger, 1970) identified Toarcian anunonites from sedimentary beds apparently belowvolcanic rocks (Francis Lake Member of current usage) and Middle Bajocian ammonites from sedimentarybeds intercalated with massive volcanics (Weaver Lake Member of current usage). Arthur (1986) and Arthur77et al. (1993) present a comprehensive review of paleontology of the formation, and interpret the formation torange from Middle Toarcian(?) to Early Bajocian(?).New biostratigraphic control is established by the discovety of well preserved Dactylloceras sp.ainmonites in the Francis Lake Member, which establishes the age of the member as late Early Toarcian (G.K.Jakobs, written communication, 1992). The existence of àrystal tuff intercalated with anunonite bearingargillite in the Francis Lake Member firmly establishes the initiation of volcanism in the Harrison LakeFormation to be late Early Toarcian (Ca. 190 Ma).B. U-Pb GeochronologyA sample of rhyolite from near the top of the Weaver Lake Member on Echo Island was collected forU-Pb zircon dating. Standard mineral separation techniques yielded abundant, high quality, clear, pale pink,moderately elongate to subequant zircon (length/width ratios of about 3.5 to 1.25). Mineral separation andanalytical procedures are outlined in Friedman and Armstrong (1994). There were no visible cores observedin any zircons from this sample. All fractions were strongly abraded to eliminate or minimize the effects of Pbloss. Analyses of four zircon fractions (A-D) from the rhyolite form a quasi-linear array which indicates aMiddle Jurassic magmatic age for the sample, and the presence of an old inherited component in fractions A,B, and C (Fig. 4.6a). Fraction D consisted of tips broken off of the most elongate zircon grains available, inorder to eliminate any possible inherited core material. This fraction is concordant and provides the bestestimate for the age of the sample of 166.0 ± 0.4 Ma (age and precision based on 206Pb/38Udate for thisfraction). The narrow error envelope assigned to this interpreted age is considered warranted because thehigh quality, strongly abraded, low uranium zircon tips from fraction D are unlikely to show the effects of Pbloss. However, if minor Pb loss has affected fraction D, a maximum possible age is give by its 207Pb/6date and associated error (165.8 ± 9.9 Ma; Table 4.1). These results clearly indicate a Late Bajocian/EarlyBathonian age for the rhyolite dome, and, together with biostratigraphic data, suggest that volcanism in theHarrison Lake Formation was active from Late Early Toarcian to Early Bathonian time. A regression line780.0274ACN 0.0266S...0‘Ga.0N• 0.02560.176 0.160 0.164B0.0265CNS...0,0.0N 0.02550.17Figure 4.6 -a) Concordia diagram for rhyolite sample recovered from the upper Weaver Lake Member on EchoIsland; b) Concordia diagram for the Hemlock Valley stock. See figure 4.2 for sample locations.WEAVER LAKE MEMBERECHO ISLAND RHYOUTE166.0 ± 0.4 Mc1721701660UPPER INTERCEPT:1.3 +0.9/-0.7 Ga207 235Pb/ U207 235Pb/ U79U-PBZIRCONANALYTICALDATAFORTHEHARRISONLAKEFORMATIONFg.ciion1Wt.LiPb2206Pb’Pb420Pb5Isotopicratios(±Ia.%)’Isotopicdatcs(?.(a.±2o)1mappmppmPbP1%Pb/U7Pb5U207Pb/PbPbf8U2°’Pb/LJ207Pb/6bEchoIslandRhyolite:293JBM92A,c.N2.p0.2461493.9253424•90.02637±0.110.1$04i0.230.04962±0)4167.1±0.4161.4±07177.2±6.4B.mN2p0)101714.82131253330.01633±0.320.3*08±0.240.04979±0.14367.5±0.4161.7±0.7315.2±0.7C,m.N2.p.cq0.0111704.617602210.70.02641±0.110.IIM0(15002±0.17361.4±0.4170.3±0.1395.9±7.1D.c.N2.p.ti0.05715*4.212321211.40.02609±0.120.1776±030004938±023166.0l0.4166.0±0.9165.1±9.9HemlockValleyStock:MV-85-HL1A.c.N1.p0.9)O3253.5324534933.40.02655±0.160.1*91±0310.053*4±0.20368.9±0.5176.4±1.0278.5±93B.f.Nl.p35003323.648636513.10.015*0±0.140.1759±0.220.04946±0.11364.2±0.4164.5±0.7169.6±5.1C,f,N2.p.e0.09413*3.12499933.70.02608±0.II0.1775±0.230.04937±0.15165.9±03165.9±0.7165.5±6.8D,m.N2.p0.1471333.624273333.101U511±0.I00)775±0.240.04975±0.36364.7±03365.9±0.73*3.2±7.4‘Allfiuonsareairabraded;Grainsize,smallestdimension:c=>134pm,m=c134and>74pm,f=c74prn;Magneticcodes:Franzmagneticseporator sideslopeatwhichgrainsarcnonmagnetic; e.g.,N1=nonmagneticat10; Fieldstrengthforallfractions‘I.8A;Frontslopeforallfracuons=20°;Graincharactercodes:c=clongate,eq=cquant, p=prismatic,ti=tips;2RadiogenicPb2MeasuzedratiocorrectedforspikeandPbfractionaiionof0.0043/amu±20%(Dalycollector)4TotaIcommonPbinanalysisbasedonbbnkisotopiccompositiondeterminedfromproceduralblanks5RadiogenicPb‘CorrectedforblankPb(5-2Opg),U(I-3pg)andcommonPb(Stacey-KramersmodelPbcompositionatthe207PW°’Pbdateoffractionorageofsample);NotethatfractionsAandBforHemlockValleyStockweredonein1990whenproceduralblankswere50andlOpgforPbandU.respectively.CD 0 C)00 0•T1through the four analyses yields a poorly constrained upper intercept of 1.3 Ga, indicating Late Proterozoicinheritance, and suggesting the incorporation of an older crustal component in Middle Jurassic magmatism(Friedman and Ciii, 1994).The age of the rhyolite dome complex is nearly identical to that of the Hemlock Valley stock, a quartzfeldspar porphyry stock that intrudes the Weaver Lake Member north of Mt. Klaudt (Fig. 4.2). A sample ofthe stock was originally collected for U-Pb dating in 1987 by J.W.H. Monger. At that time, two relativelyimprecise analyses of unabraded zircon determined by P. van der Heyden indicated a crystallization age ofabout 165 Ma (R. Friedman, personal communication, 1994). Zircon fractions analyzed from this sampleduring the present investigation consist of moderate to good quality, clear to slightly cloudy grains withsubequant to moderately elongate prismatic morphology (length/width ratios of about 1.5 to 3.0). No coreswere visible. Four new analyses of abraded zircon fractions are shown on figure 4.6b. The data indicate thepresence of vaiying amounts of inheritance in fractions A, B and D, with superimposed Pb loss on fractions Band D. Pb loss cannot be ruled out for fractions A and C. Fraction C, which consisted of abraded, relativelyelongate prismatic grains, is concordant at 165.9 Ma and provides the basis for the interpreted age estimate of165.9 +6.41-0.3 Ma. The maximum age is based on the 207Pb/6date and the minimum age on the206Pb/38Udate, both for fraction C. The larger upper error envelope takes into account the possibility of Pbloss that has not been completely eliminated through abrasion.The similarity of magmatic age, inheritance characteristics, and Nd and Sr isotopic values (seefollowing isotopic discussion) strongly argues the dome and stock are comagmatic. In addition, the westernmargin of the Harrison Lake Formation is intruded by the Mt. Jasper pluton, a massive tonalite/granodioriteintrusion. The Mt. Jasper pluton yields a 167 +1-4 Ma U-Pb zircon age (Friedman and Armstrong, 1994),which overlaps the age of both the rhyolite dome and the quartz feldspar intrusion, suggesting the granitoidrocks intruding the Harrison Lake Formation are simply the subvolcanic roots to the Middle Jurassic volcanicarc.814.6 STRUCTURAL DEFORMATIONThe Harrison Lake Formation is deformed into a broad, gentle west-northwest trending, shallowlywesterly plunging anticline (Fig. 4.2). Limited continuous exposures suggest the formation forms gentlydipping homoclines. The contact between the Harrison Lake Formation and the overlying Mysterious CreekFormation exposed on the south shore of the Harrison River appears disconformable. However, new mappingin the Harrison Lake Formation conducted during this investigation documents mesoscale folds in the WeaverLake and Echo Island Members throughout the outcrop belt. Mesoscale folds are of 1-10 m amplitude,generally have near E-W fold axes, and are dominantly north vergent. Bedding on the north and northeastflanks of Mt. McRae is locally overturned to the south.Poor exposure makes adequate doctimentation of the deformational history of the Harrison LakeFormation difficult. Folding about northwest-trending fold axes was documented by Pearson (1973). Arthur(1987) described close mesoscale folds with west-northwest-trending fold axes on the east side of Echo Island,together with EW-trending gentle folds and near-vertical bedding on the northwest flank of Mt. McRae. Thechaotic distribution of structural attitudes throughout the outcrop belt, locally evident mesoscale folds, andoverturned bedding in the Echo Island Formation contrast sharply with the unfolded, gentle, north to northeastdipping beds of the overlying Mysterious Creek Formation. The contrast in structural styles between the twoformations strongly suggests the presence of an angular unconformity between the two, and indicates a post-Early Bathonian (Ca. 165 Ma, pre-Early Callovian (Ca. 160 Ma) compressional event in the region. Thetiming of this deformational event corresponds to the timing of isoclinal folding in the Bowen Island Group tothe west, which is constrained to be post-185 Ma, pre-155 Ma (Friedman et al., 1990; Monger and Journeay,1994).824.7 GEOCHEMISTRYGeochemical analyses are reported for a representative suite of primaiy volcanic rocks fromthroughout the entire stratigraphic range and lateral extent of the Weaver Lake Member. The suite comprises13 extrusive volcanic rocks, 4 dikes, and one sample of a coeval quartz feldspar porphyiy (Fig. 4.2). Sampleselection was limited to relatively unaltered samples of andesitic to dacitic composition. Homogenized rockpowders were prepared from 2-5 kg whole rock samples, with care taken to remove weathered surfaces andsecondary veins. Major and minor element concentrations were detennined by fused disc X-ray fluorescence;trace and rare earth elements were measured by inductively coupled mass spectrometiy. Complete samplepreparation and analytical techniques are given in Appendix A.Geochemical results are also summarized from a detailed companion study of lithostratigraphy,geochemistiy and mineralization of the Seneca area by Sean McKinley (Fig. 4.2). Data for 54 samples fromthe Seneca area indicate a bimodal volcanic suite characterized by a maflc suite (Si02=45-53 wt %; n=14) anda felsic suite (Si02=66-74 wt %; n=40; Fig. 4.2; Table 4.2).Primary volcanic rocks in the Weaver Lake Member range from basaltic andesite to rhyolite (Table4.2; Fig. 4.7). There is a general geochemical trend from mafic compositions near the bottom of the memberto more felsic compositions near the top of the member. This trend is supported by field evidence thatdocuments basaltic andesite to andesite near the base of the Weaver Lake Member, dacite comprising themajority of the member, and an increase in the proportion of rhyolite within the upper portions of the member.The increase in pyroclastic material evident in the Echo Island Member suggests an increase in explosivevolcanism near the top of the formation, which is consistent with an increase in silica and probably volatilecontent of the system.Major and trace element values indicate the rocks are of calc-alkaline magmatic affinity, and rangefrom low- to high-K types (Fig. 4.7). However, elemental scatter on Harker variation diagrams suggests83TotalWITrace Elements96.652.75Sample 43BM92 116JBM92 122JBM92 123JBM92 142JBM92 150JBM92 151JBM92 287JBM92 291JBM92 292JBM92Si02 61.68 58.12 75.29 68.40 56.32 72.99 74.74 63.00 67.51 74.68Ti02 0.865 0.765 0.290 0.505 0.830 0.370 0.315 0.67 0.510 0.250A1203 16.76 19.16 12.83 15.73 19.23 14.05 13.47 15.90 15.58 14.36FeO* 7.30 6.98 1.50 4.18 9.56 2.49 2.96 5.13 4.19 2.15MnO 0.15 0.20 0.05 0.09 0.18 0.11 0.10 0.10 0.09 0.05CaO 5.03 2.37 1.51 1.61 3.16 2.07 0.63 2.45 1.20 5.12MgO 2.54 4.62 0.45 2.46 5.82 1.05 1.03 2.22 2.54 0.80K20 0.54 1.54 3.66 3.67 2.56 3.04 1.19 4.78 4.09 1.38Na20 4.56 5.95 3.49 3.69 1.75 3.20 5.27 2.19 4.03 0.82P205 0.25 0.21 0.07 0.17 0.22 0.10 0.07 0.21 0.15 0.05100.49 100.68 99.29 100.96 100.67 99.74 100.09CrScNi 4 • 9 - S100.36 99.9010 14 15 13 25 12 17 16 17 10V 95 170 14 77 221 25 4 83 76 11Ba 185 658 1438 1189 424 1488 395 838 1175 616Rb 7.31 28.81 36.79 54.40 60.82 41.95 12.60 32.64 52.08 30.10Sr 147 231 147 244 33 163 153 289 121 219Zr 68 128 136 147 106 138 149 131 154 125Y 30.40 23.60 19.14 23.26 21.72 22.16 56.20 33.57 24.85 19.01Nb 2.67 4.98 5.40 6.02 4.48 5.68 5.32 6.62 6.49 5.44Ga - - - - - - - -Cu 19 7 - 8 34 3 - 9 26 2Zn 88 108 46 53 117 52 62 68 88 45Pb 2.32 3.40 1.37 4.18 1.92 3.59 1.58 3.91 17.55 7.73La 7.37 14.33 g.03 17.85 12.60 15.42 15.32 15.66 16.47 14.16Ce 15.73 27.75 18.72 28.71 24.31 25.04 29.73 30.56 27.24 24.98Pr 2.34 3.36 2.30 3.40 3.02 3.12 4.66 3.95 3.28 2.80Nd 11.81 14.92 9.99 13.86 13.34 12.70 21.95 17.82 13.66 10.93Sm 3.83 3.90 2.49 3.20 3.59 3.05 6.63 4.91 3.38 2.63Eu 1.43 1.15 0.69 0.98 1.02 0.80 1.69 1.35 0.93 0.65Gd 4.26 3.85 2.57 2.98 3.56 2.97 7.21 5.01 3.38 2.41Th 0.82 0.67 0.48 0.56 0.63 0.54 1.42 0.90 0.59 0.46Dy 5.45 4.21 2.96 3.64 4.04 3.43 9.27 5.87 3.92 3.01Ho 1.18 0.88 0.67 0.80 0.83 0.77 1.98 1.25 0.84 0.65Er 3.45 2.69 2.01 2.38 2.46 2.33 5.97 3.72 2.58 2.07Tm 0.48 0.37 0.30 0.35 0.34 0.34 0.83 0.53 0.36 0.32Yb 2.98 2.37 2.00 2.31 2.25 2.31 5.42 3.44 2.46 2.00Lu 0.49 0.40 0.36 0.40 0.38 0.39 0.92 0.57 0.44 0.36Hf 1.82 2.85 3.11 3.39 2.68 3.18 3.68 3.16 3.14 3.08Th 0.71 3.23 2.61 3.98 2.97 2.45 1.12 1.94 3.39 2.57U 0.25 0.98 0.83 1.31 0.96 0.79 0.44 0.66 1.10 1.00Ta 0.20 0.43 0.42 0.49 0.39 0.48 0.38 0.43 0.52 0.54Cs 0.36 0.18 0.48 0.38 0.94 2.38 0.10 1.21 0.15 2.50Ce/Yb 5.3 11.7 9.4 12.4 10.8 10.8 5.5 8.9 11.1 12.5Ba/La 25.1 45.9 159.2 66.6 33.7 96.5 25.8 53.5 71.3 43.5La)Th 10.4 4.4 3.5 4.5 4.2 6.3 13.7 8.1 4.9 5.5La/Nb 2.8 2.9 1.7 3.0 2.8 2.7 2.9 2.4 2.5 2.6Ba/Nb 69.3 132.1 266.3 197.5 94.6 262.0 74.2 126.6 181.0 113.2Zr/Y 2.2 5.4 7.1 6.3 4.9 6.2 2.7 3.9 6.2 6.6Normalized ValuesLaiYb 1.6 4.0 3.0 5.1 3.7 4.4 1.9 3.0 4.4 4.7La/Sm 1.2 2.2 2.2 3.4 2.1 3.1 1.4 1.9 3.0 3.3Table 4.2a- Geochemical data for the Harrison Lake Formation.84Sample 293JBM92 306JBM92 3073BM92 13JBM93 143BM93 15JBM93 173BM93 21JBM93 22JBM93 253BM93Si02 73.44 79.57 68.11 75.52 77.39 67.44 70.40 68.11 77.88 74.53Ti02 0.250 0.200 0.610 0.248 0.228 0.592 0.512 0.541 0.164 0.299A1203 15.08 11.35 15.60 13.38 13.37 15.53 14.73 14.81 12.55 13.53FeO* 1.20 1.38 4.45 1.30 1.32 3.86 4.58 4.30 1.42 1.91MnO 0.04 0.03 0.06 0.03 0.04 0.08 0.17 0.12 0.05 0.05CaO 3.80 0.43 3.62 2.22 1.19 1.81 1.28 3.03 0.87 1.03MgO 0.68 0.45 1.77 0.72 0.57 1.10 1.14 1.35 0.35 0.76K20 3.27 2.91 1.62 0.24 0.25 3.89 0.07 1.49 1.63 3.33Na20 1.55 3.36 4.12 5.11 6.67 5.62 7.41 4.56 5.65 4.57P205 0.05 0.06 0.21 0.05 0.05 0.22 0.15 0.16 0.03 0.07Total 99.49 99.88 100.65 98.82 101.08 100.14 100.43 98.48 100.59 100.08WITrace ElementsNi - - 8 9 4 5 7 8 8Cr 13 16 12 3 5 4 0 2 3 4Sc - - 11 6 17 29 18 3 7V 10 11 43 28 13 53 0 45 4 24Ba 1190 1060 154 239 91 937 71 667 940 929Rb 83.87 81.81 16.36 1.40 2.55 57.40 1.12 23.49 20.42 37.80Sr 163 113 72 242 200 161 123 339 147 152Zr 124 93 111 115 112 137 87 130 123 128Y 19.40 13.98 31.08 21.01 13.94 36.08 36.85 31.61 23.22 21.24Nb 5.44 6.31 3.38 2.95 3.47 7.22 3.87 7.00 5.36 4.77Ga - - 12 9 14 15 13 9 13Cu - - 6 6 13 4 5 8 8Zn 38 35 75 8 11 64 92 69 30 32Pb 8.07 1.55 2.53 0.84 0.69 6.50 1.42 4.51 2.32 2.27La 12.65 16.50 11.55 11.10 10.66 18.38 7.38 12.46 14.24 14.42Ce 24.62 28.13 22.86 22.24 19.99 31.50 17.06 24.93 26.51 23.89Pr 2.84 3.01 3.31 2.81 2.32 4.45 2.53 3.25 3.07 3.07Nd 11.59 11.15 15.41 11.77 9.01 19.59 12.52 15.16 12.74 12.27Sm 2.72 2.11 4.59 3.02 2.16 5.19 4.06 4.53 3.11 2.99Eu 0.62 0.57 1.46 0.68 0.57 1.37 1.39 1.25 0.69 0.77Gd 2.76 1.89 4.67 2.82 1.83 5.16 4.96 4.50 2.88 2.90Th 0.51 0.32 0.84 0.51 0.32 0.95 0.97 0.84 0.53 0.54Dy 3.25 2.07 5.48 3.30 2.14 5.98 6.43 5.35 3.68 3.25Ho 0.68 0.42 1.17 0.72 0.46 1.25. 1.44 1.15 0.81 0.65Er 2.07 1.34 344 2.25 1.47 3.79 4.27 3.45 2.48 2.11Tm 0.29 0.21 0.48 0.33 0.22 0.51 0.60 0.49 0.37 0.32Yb 2.00 1.43 3.02 2.26 1.63 3.36 3.90 3.20 2.67 2.09Lu 0.34 0.27 0.47 0.40 0.28 0.54 0.65 0.53 0.49 0.35Hf 3.40 2.41 2.68 2.97 2.76 3.41 2.63 3.27 3.40 3.05Th 3.08 5.30 1.25 1.87 1.56 2.59 0.70 1.76 2.39 2.43U 1.13 1.77 0.43 0.70 0.52 0.96 0.26 0.60 0.91 0.83Ta 0.58 0.63 0.21 0.20 0.22 0.43 0.21 0.39 0.37 0.30Cs 1.73 0.34 1.18 0.07 0.05 0.12 0.01 0.83 0.64 0.25Ce/Yb 12.3 19.7 7.6 9.8 12.3 9.4 4.4 7.8 9.9 11.4Ba/La 94.1 64.2 13.3 21.5 8.5 51.0 9.6 53.5 66.0 64.4LafI’h 4.1 3.1 9.2 5.9 6.8 7.1 10.5 7.1 6.0 5.9La/Nb 2.3 2.6 3.4 3.8 3.1 2.5 1.9 1.8 2.7 3.0Ba/Nb 218.8 168.0 45.6 81.0 26.2 129.8 18.3 95.3 175.4 194.8Zr/Y 6.4 6.7 3.6 5.5 8.0 3.8 2.4 4.1 5.3 6.0Nonnalized ValuesLa/Yb 4.2 7.6 2.5 3.2 4.3 3.6 1.2 2.6 3.5 4.5La/Sm 2.8 4.8 1.5 2.2 3.0 2.2 1.1 1.7 2.8 2.9Table 4.2b - Geochemical data for the Harrison Lake Formation.85FeO’Figure 4.7- a) AFM (FeO-Na20+K-MgO) diagram illustrating strong calcaikaline affinity for WeaverLake Member volcanic rocks. From Irvine and Baragar (1971); b) ZrITiO2vs. Si02 compositionaldiagram for Weaver Lake Member volcanic rocks, showing a compositional range from basalticandesite to rhyolite. From Winchester and Floyd (1977); c) Nb vs. La trace element diagram for WeaverLake Member volcanic rocks, demonstrating medium- to high-K calcalkaline affinity of rocks. FromGill (1981).(a)MgO807570(b)6560Na20 + K20I55504540BastTrach/NephII I.001 .01 .1 10Zr/Ti02(c)0 5 10Nb (ppm)86post-depositional mobilization of K, Rb, and Ba (Fig. 4.8a); thus interpretations based on these elementsshould be used with caution. Although mobility of K, Rb, Ba, and to a lesser extent, Ca, Na, and Mn isindicated on Harker diagrams (Fig. 4.8a), linear to curvilinear trends in Al, Fe, Mg, Ti, V, andP205 againstSi02 (Fig. 4.8b) suggest that at least part of the original volcanic fractionation trends has been preserved.Evidence of elemental mobility casts doubt on analysis of any mobile constituent, including Si02,andit is therefore necessary to examine relatively immobile components to assess fractionation trends.Investigations of hydrothermal alteration of volcanic rocks demonstrate that Al, Ti, the high field strengthelements (HFSE; especially Zr, Y, Nb), and generally the REE are immobile, and are therefore useful inmonitoring fractionation trends (MacLean and Barrett, 1993; Barrett and MacLean, 1993). However,examination of incompatible-compatible element pairs (Zr/Si02,Zr/Al23,ZrJTiO2,Y/Si02)in the HarrisonLake Formation indicate nonuniform behavior of some of HFSE (Zr, Y, Nb), suggesting that these elementsbehaved compatibly in the more felsic end of the compositional range. It is important to note that thiscompatible behavior is not a function of alteration, but is common in calcalkaline systems (MacLean andBarrett, 1993). It is therefore necessary to use immobile compatible elements (Al, Ti, Sc, V, and Cr) tomonitor the evolution of the Harrison Lake Formation rock series. The immobile compatible element pairTiO2IAlO3yields a well-defined linear trend consistent with a smooth fractionation sequence with minimalalteration effects (Fig. 4.9a). The two high Al data points are plagioclase-phyric samples. Three felsicsamples plot off the fractionation trend along a linear trend through the origin, indicating alteration of thesesamples (292JBM93, 2923BM93, 306JBM93; MacLean and Barrett, 1993). The fractionation trend defined bythe majority of the samples is supported by the linear relationship between Ti02 and Si02, a relationship thatalso suggests minimal mobility of 5i02 (Fig. 4.9b). Significant gain or loss of silica would pull the data pointsaway from their unaltered precursor composition on the fractionation trend, along lines oriented toward apoint at 100 wt % Si02 and 0 wt % Ti02, reflecting enrichment or depletion of Ti02 relative to 5i02. Notethat the three samples displaying alteration on theTi02/A103diagram also show a similar deviation fromthe Ti02/Si trend, that is consistent with silica mobility in these samples.870CC)Si02 (wt%) S102 (wt%)Figure 4.8- a) Major and minor element Harker variation diagrams for Weaver Lake Member volcanic rocks,illustrating elemental scatter; b) Minor and trace element Harker variation diagrams for Weaver LakeMember volcanic rocks. Linear trends indicate a lack of elemental remobilization and suggest a geneticrelationship between samples. Enclosed fields represent samples from the Seneca area..S., : •.. + +o.20150—\ 100800.6040200.10201 5002009006003000:•—... * 8...00CD0I... ,•, ...•e108642021 .510.50300,..••.• •.200•‘ 100>06420864200.30.20. 10...••• •.,0Cz0C‘4, 10••..,.,40 50 60 70 802502000. 15010050040cc%50 60 70 808820..181614fractionationinferred8 /inferredtrendalteration6 trend4200.1 .2 .4 .5 .6 .7 .8 .9 1.0Ti0(wt%)1009050inferred40 fractionation30 trend201000 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0Ti02(wt%)Figure 4.9- a) Ti02 vs. A1203 immobile compatible element diagram. Note well-defined linear fractionationtrend. Altered samples (open boxes) plot on linear array trending toward origin; b) Ti02 vs. Si02diagram. Note linear fractionation trend, and slight displacement of altered samples.89Linear arrays ofA1203,MgO, Ti02 V andP205 on Harker diagrams are consistent with a simplefractionation trend within volcanic rocks of the Weaver Lake Member. Several of the patterns arecharacterized by an inflection point between 59-63 wt % Si02,which is inferred to represent the cessation ofcrystallization of ferromagnesian phases, and the onset of felsic mineral crystallization. Data from the Senecaarea partially overlap the fractionation trends suggested by the regional data set. The Seneca data appear tooutline distinct mafic and felsic subgroups, with a compositional gap between 53-66 wt % Si02.Thecompositional groupings in the Seneca area could be used to infer a bimodal distribution of the volcanic rocksof the Weaver Lake Member. However, the entire sample set in the Seneca area is derived from less than 400m of strata within a 6 area, so caution is necessary in deriving regional implications from such an areallyand stratigraphically limited data set. Further sampling is required to determine if these subgroups areregionally consistent.Trace element values for the Weaver Lake Member on the Hf-Th-Ta tectonic discrimination diagramof Wood (1980) indicate the suite is of calcalkaline affinity, and was developed within a destructive platemargin (Fig. 4.10). Incompatible trace element spider diagrams and REE plots are also consistent with amedium K calcalkaline magmatic affinity (Fig. 4.1 la,b). The incompatible trace element diagram (Fig. 4.1 la)displays an overall enrichment of large ion lithophile (LIL) elements relative to HFSE and the strongly spikedpattern characteristic of subduction-related magmatism (Thompson et al., 1984; Wilson, 1988). Volcanicrocks of the Weaver Lake Member are enriched in low field strength elements (LFSE) relative to primordialmantle values. The irregularity among samples in this portion of the diagram may reflect the mobilization ofthe LFSE elements suggested by the Harker diagrams (Fig. 4.8a). Weaver Lake Member volcanic rocks aredepleted in Nb, Sr, and Ti, and enriched in Zr (Fig. 4.1 la). Depletion of Nb and Ti, together with lowabundances of Ta and high LaJNb ratios of 1.5-2.0 (Table 4.2), is similar to that of modern calcalkalinesystems, and may be the result of partial melting and early fractionation of Ti-bearing phases (Pearce, 1982;Thompson et al., 1984). Early fractionation of Ti-bearing phases is supported by a steady decrease in Tiabundances with increasing silica (Table 1, Fig. 4.9a). The marked depletion of Sr is attributed to plagioclasefractionation, which is also suggested by Eu anomalies in REE plots (Fig. 4.1 ib).90ThHf/3TaFigure 4.10- HE’3-Ta-Th tectonic discrimination diagram for Weaver Lake Member volcanic rocks. Samplesplot within destructive plate margin (subduction-related) field (Field D). From Wood (1980). Field A -normal MORB; Field B - enriched MORB; Field C - within plate basalt.9110001000a)aEC,)0.1100-I-c010ci)a0C,)•1La Ce Pr Nd Sm Eu Gd Th Dy Ho Er Tm Yb LuFigure 4.11- a) extended trace element abundance spider diagram for Weaver Lake Member volcanic rocks.Note scatter in LFSE (Low Field Strength Elements) at left side of diagram, pronounced troughs at Nb,Sr, Ti, and peaks at Zr. Normalized to MORB values, using data from Taylor and McLennan (1985); b)Rare earth element diagram for Weaver Lake Member volcanic rocks. Note LREE enrichment andslight negative Eu anomaly. Normalized to chondritic values, using data from Sun (1982).Cs Ba U Nb Ce Nd Zr Eu Gd Y YbRb Th K La Sr Hf Sm TI Dy Er Lu(b)92Rare earth element plots display LREE enrichment (La/Yb = 1.5-4.25), characterized by positiveLa/Sm values (1.3-2.25), slight negative Eu anomalies, and flat to slightly depleted HREE patterns (Fig. 4.1 ib,Table 1). These patterns are consistent with those of modern medium K calcalkaline suites (Gill, 1981;Wilson, 1988). Total REE values range from 60-115 ppm (Table 4.2), reflecting the slightly enrichedcharacter of the suite. One andesitic dike sample (4JBM92) shows an anomolously fiat pattern relative to therest of the suite.4.8 Nd-Sr ISOTOPIC SYSTEMATICSNd and Sr isotopic analyses of a representative suite of Weaver Lake Member volcanic rocks serve toconstrain magma source area characteristics, whereas analyses of fine-grained sedimentary rocks fromthroughout the formation constrain the nature of the sediment provenance (Figs. 4.12, 4.13). Isotopic data arepresented in Table 4.3, and isotopic values are plotted on an initial eNd vs initial 87Sr/6rdiagramcontaining comparative fields and against stratigraphic position (Figs. 4.12, 4.13). Isotopic procedures aredetailed in Appendix A.The Nd and Sr isotopic values of Weaver Lake Member volcanic rocks attest to the juvenile characterof the magma source region. The majority of the volcanic rocks have CNd values of +4•5.. +6, and initial87Sr/6r ratios of 0.7035- 0.7050. This restricted range of isotopic values leads to a tight cluster of datapoints on a ENd versus 87Sr/6rdiagram, and attests to the isotopic homogeneity of an uncontaminated,depleted mantle-derived magma system (Hawkesworth et al, 1993). The 6Nd and87Sr/6rvalues are onlymoderately displaced from depleted mantle values (MORB field of figure 4.12), and plot well within theuncontaminated island arc volcanic (IAVu) field. The juvenile isotopic values are consistent with traceelement data, particularly the low Ce/Yb (<15) ratios, which have been used to argue for magma derivationfrom an only slightly enriched mantle wedge (McCulloch and Gamble, 1991; Hawkesworth et al., 1993; Fig.4.10 and 4.12, Table 4.3). Minor enrichment of the magma source may be attributed to slab-derived elementalflux, subducted sediment, or minor crustal contamination (von Drach et al., 1986). Isotopic values of volcanic9310.00CDIIFigure 4.12-versus 87Sr/6r for Harrison Lake Formation rocks. Note tight cluster of volcanic rockvalues (square symbols), and their affinity with uncontaminated volcanic arc field (IAVu). Scatter ofsedimentary rock samples (circular symbols) is attributed to detrital mixing ofjuvenile volcanic detritusand a component of more evolved (continental?) detritus. Samples plotting to left of mantle arraysuggest local disturbance of Rb/Sr system. IAVc = contaminated island arc volcanic rocks; MORB =mid-ocean ridge basalt; CHUR = Chondritic Uniform Reservoir. Lake FormationI.Sedimentaty RocksVolcanic Rocks.1IIINdL4VuS..CHUR2.000.00-200 --4.00-6.00IAVc .I-8.00-10.000.7020ToPrecambrianCrust0.7040 0.7060I0.708087S/8óSr0.710094MapAge87Sr6r(m)±IDHarrisonLakeFormationIsotopicDataTableTable4.3RbSrRb/Sr87Rb/6Sr87Sr/6r(i)SmNdSm/Nd143Nd/4d±Nd(O)SNd(i)HarrisonLakeFormationSediments961BM9211800.70682044207.1380.00.381.110.7039752.6611.510.14000.5128070.0000083.304.60102JBM9221800.70637615231.1423. from the Harrison Lake Formation overlap those reported by Friedman et al. (1994) for Mesozoicplutons in the southern Coast belt. The volcanic rocks have a similar range of eNd, but display moreradiogenic87Sr/6, which is interpreted to be the result of seawater alteration. One volcanic sample(307JBM92) has a very low initial 87Sr/6r(—4).7026), and plots significantly to the right of the mantlearray. This sample also has an anomolously low Rb concentration, which may indicate post-depositionaldisturbance of the Rb/Sr system.Nd and Sr isotopic values for fine-grained sedimentaiy rocks of the Harrison Lake Formation arewidely distributed, but generally plot within the IAV field (Fig. 4.12). The isotopic values of the sedimentaryrocks are displaced from those of volcanic rocks, and are characterized by a broader range of87Sr/6randoverall lower Nd values. The shift to more radiogemc87Sr/6rand lower CNd may reflect variable degreesof isotopic mixing between volcanic detritu and intrabasinal authigenic sediment or perhaps an extrabasinalevolved component. The lack of identifiable pelagic or carbonate sediment in the Harrison Lake Formationsuggests an influx of extrabasinal evolved detritus is most probable. Low values of87Sr/65 are interpretedto result from disturbance of the Rb/Sr isotopic system.A plot of isotopic values against stratigraphic position suggests a temporal control on the distributionof isotopic values, particularly ENd (Fig. 4.13). The most evolved ENd values occur in fine-grained marinesediments of the Francis Lake Member. There is a significant shift in ENd values within the member, fromapproximately +45 to about +2 in the Late Toarcian. This shift occurs during a time of high volcaniclasticinflux into the basin (average ENd values +45 6), and thus requires addition of isotopically evolved detritusto account for the lowering of ENd values. This influx of an evolved component was brief, however, becauseby the mid-Aalenian, both sedimentary and volcanic rocks again display juvenile values (eNd = 45.. 6)characteristic of the middle Jurassic arc (Fig. 4.13). The upper portion of the formation is isotopicallyheterogeneous compared to the majority of the formation. This increase in heterogeneity may have been theresult increased rhyolitic volcanism coupled with the contamination evident in the U-Pb systematics.96I’ ft CD .1. ii 03C C—UICoa’ C 00 C—4.9 ALTERATIONThe Harrison Lake Formation displays a low-grade alteration assemblage dominated by minerals ofthe prehnite-pumpellyite fades. Alteration assemblages include, in decreasing order of abundance, calcite,epidote, hematite, zeolites, quartz, and rarely albite. Alteration varies nonsystematically with geographiclocation and stratigraphic position throughout the outcrop belt. Silicification of volcanic rocks is common.Contact metamorphism adjacent to Middle Jurassic intiusive bodies northwest of Mt. Klaudt and along thewestern margin of the formation is limited to thin aureoles of hornblende homfels, silicification andpyritization, suggesting that these intrusions were not responsible for regional alteration. Syndepositionalalteration is suggested by the incorporation of silicified lithic clasts in tuffbreccia with unaltered lapilli tuffmatrix, and by altered lava flows and volcanic breccias cut by unaltered comagmatic dikes. Localizedalteration and the presence of relatively unaltered subvolcanic Triassic basement rocks implies the possibilityof a structural control focusing the flow of alteration fluids.4.10 MUERALIZATIONThe Harrison Lake Formation hosts Kuroko-style stratiform massive suffide bodies, as well asstockwork and vein mineralization in the Seneca Zn-Cu-Pb mineralized area in the southwestern portion of theoutcrop belt (Urabe, 1983; Fig. 4.2). Host rocks consist of andesite and dacite lava and volcanic breccia,volcaniclastic sedimentary rocks, and abundant synvolcanic dikes and sills of andesitic to rhyoliticcomposition. Alteration of the host rocks is minor, and generally consists of minor silicification andsericitization; primaiy volcanic and sedimentologic textures are well preserved. Adjacent to mineralized zonesalteration is more intense, and consists of moderate to intense silicification and sericitization and lesserepidote -calcite alteration. McKinley et al. (1994) describe three types of mineralization in the area: 1) semi-massive to massive sulfide bodies, consisting of stratiform lenses of sphalerite, pyrite, and chalcopyrite withlesser galena hosted by fragmental volcanic rocks; 2) semi-massive and disseminated sulfides and barite98associated with altered dacitic epiclastic conglomerate; and 3) stockwork and stringer quartz-sulfidemineralization consisting of veins of sphalerite, pyrite, and chalcopyrite in altered dacite lava.Stratabound mineralization in the Seneca district is interpreted to be preferentially located near thecontact between the Weaver Lake and Echo Island Members of the Harrison Lake Formation. Thisinterpretation is based on: 1) the close spatial relation between stockwork mineralization and rhyolite andquartz feldspar porphyiy intrusions; 2) the abundance of sedimentaiy rocks in the Seneca area; coupled withthe presence of the Weaver Lake/Echo Island Member contact to the southeast (Fig. 4.2); and 3) the regionalconcentration of disseminated suffide mineralization in the upper portions of the Weaver Lake Member,particularly in the area near Mt. McRae. Dating of the felsic flows and sills in the Seneca area is needed toconfirm this interpretation.4.11 EVOLUTION OF HARRISON LAKE FORMATIONAccumulation of the Harrison Lake Formation is interpreted to have begun in the Early Toarcian withdeposition of conglomerate of the Celia Cove Member. The only age control on the Celia Cover Member is itsstratigraphic position, unconformably above Middle Triassic rocks of the Camp Cove Formation, andconformably below late Early Toarcian shale of the Francis Lake Member (Arthur, 1987; Arthur et al., 1993).However, the gradational contact between the Celia Cove and Francis Lake members, together withsimilarities in provenance and style of sedimentation, suggest the Celia Cove Member is roughly coeval with,or only slightly older than, the Early Toarcian Francis Lake Member.The basal conglomerate of the Celia Cove Member contains angular clasts of chert, argillite, andgreenstone derived from inunediately subjacent lithologies of the Camp Cove Formation. The sharpness of thecontact, angularity of the clasts, local clast derivation, and matrix supported nature of the conglomerate arguesfor limited transport distance and rapid deposition in a tectonically active environment. The basalconglomerate grades upward into a more polymict conglomerate containing well-rounded clasts of andesite99porphyry, dacite porphyry, chert, volcanic lithic arenite, and rare well-rounded bioclastic limestone clasts.The heterogeneity of the clasts, higher degree of rounding, the better sorting and stratification, and the higherproportion of sandstone interbeds in the upper portion of the Celia Cove Member suggest derivation from amore distal heterolithic source area. The lack of sedimentary structures indicative of traction currents, matrixsupported nature of the conglomerate, and the cyclic depositional pattern indicate deposition by a migratingdistributaiy system in a marine environment below effective wave base, probably in the mid- to lower fanregion of a submarine fan complex (Walker, 1978).Coarse clastic rocks of the Celia Cove Member grade upward into sandstone, siltstone and argillite ofthe Francis Lake Member, culminating in a 15-20 m thick ammonite-beanng argillite succession that caps thefining upward sequence. The fining upward sequence is gradationally overlain by crystal tuff, lapilli tuff, andcoarse lithic wacke (Fig. 4.3). The identification of primary pyroclastic deposits in the Francis Lake Memberhas important implications for the timing and mode of the initiation of volcanism in the Harrison LakeFormation. The transition from fine-grained siltstone and mudstone in the lower portion of the Francis LakeMember to medium to coarse grained lithic wacke, crystal vitric tuff and lapilli tuff interbedded with siltstoneand mudstone in the upper portion of the member is critical, as it marks the initiation of volcanism in theHarrison Lake Formation. Age constraints provided by the documentation of the ammonite generaDactylloceras and Hildaites? in the lower Francis Lake Member indicate that explosive volcanism began inthe Harrison Lake Formation by at least the late Early Toarcian (ca. 185 Ma). The increasing proportion oftuffaceous lithic feldspathic wacke, crystal tuff and lapilli tuff in the upper portions of the Francis LakeMember suggest that the frequency and intensity of pyroclastic activity increased throughout the Toarcian andEarly Aalenian, perhaps related to an increased proximity to a developing volcanic edifice.Nd and Sr isotopic analyses of fine-grained sediments and volcanic rocks within the Harrison Lake+ +Formation suggest variation in provenance, from an essentially juvemle (eNd = 4.5 to 5) source in the CeliaCove and lower portions of the Francis Lake Members, to a slightly evolved (eNd = +2 to +33) signature inthe upper portion of the Francis Lake Member, then back to a distinctly juvenile (eNd = 45 to 6)100composition in the majority of the formation (Fig. 4.13). The shift to slightly more evolved Nd values isinferred to record the influx of an evolved component (continental detritus?) into the basin in the LateToarcian, prior to the main phase of effusive volcanism in the Harrison Lake Formation.A change in eruptive style is indicated by the transition from pyroclastic and epiclastic rocks of theFrancis Lake Member to lava flows and tuffbreccias of the Weaver Lake Member in the mid-Aalenian. In theWeaver Lake Member, the cyclic interbedding of epiclastic and resedimented pyroclastic debris with primaryflows and volcanic breccias records intermittent volcanic activity alternating with clastic reworking ofpyroclastic detritus and older volcanic deposits. The presence of hyaloclastite, mass sediment gravity flows,air fall or waterlain tuffs, peperitic textures and “fire fountain” facies in drill core (McKinley et al., 1994), andthe abundance of fine-grained, parallel laminated siltstone and mudstone suggest the Weaver Lake Memberwas deposited in a subaqueous setting, below effective wave base. However, the presence of well-roundedpolymict conglomerate and channels indicates initial subaerial fluvial reworking, and local trough crossbedding, bioclastic debris (Pearson, 1973), and abundant comminuted wood debris at the base of masssediment gravity flows suggests shallow water reworking proximal to a landmass. Vent proximity is indicatedby volcanic agglomerate, thick volcanic breccias and tuff breccias interpreted as debris flows, and theabundance of sills and dikes.There is a general compositional trend evident in primary volcanic rocks of the Weaver LakeMember. Flows in the lower portion of the Weaver Lake Member are basaltic andesite and andesite, and thereis an overall increase in silica content upward. The majority of the member is dacitic in composition, whereasthe upper portion of the member becomes distinctly rhyolitic, typified by the rhyolite dome complex on EchoIsland and the abundant felsic flows, dikes, and sills in the Seneca area (McKinley et al., 1994). Kuroko-typemassive suffide deposits in the Seneca area are associated with felsic volcanics, and are therefore interpreted tobe stratigraphically located near the Weaver Lake/Echo Island Member contact.101The transition from flows and breccias in the Weaver Lake Member to well-stratified pyroclastic andresedimented pyroclastic debris in the Echo Island Member records the gradual cessation of effusive volcanismand onset of pyroclastic activity. The Echo Island Member is characterized by background sedimentationconsisting of thin to medium bedded lithic wacke, tuffaceous siltstone and mudstone that is episodicallyinterrupted by thick-bedded to massive lapilli tuffs, tuffs, and mass sediment gravity flows of resedimentedpyroclastic debris. These episodic influxes are interpreted to represent pyroclastic eruptions that flood thedepositional basin with fresh juvenile volcanic debris. These periodic clastic inundations lead to rapidaggradation of resedimented pyroclastic debris, which apparently caused dramatic shallowing of the basin,resulting in the reworking of upper portions of mass sediment gravity flows and progradation of conglomeraticchannels.The top of the Echo Island Member is interpreted to be an angular unconformity with the overlyingMysterious Creek Formation. Evidence of post-Early Bathonian, pre-Early Callovian deformation of theHarrison Lake Formation includes variable bedding attitudes, mesoscale folds, and locally overturned stratathat are absent in the overlying Mysterious Creek Formation (Fig. 4.2).4.12 MODEL FOR VOLCANIC ARC DEVELOPMENTThe stratigraphic evolution of the Harrison Lake Formation conforms to a simple model of oceanicarc development as proposed by Larue et al. (1991). The evolution from a basal locally derived conglomerateof the Celia Cove Member into fine-grained, deep water facies and overlying tuffaceous clastic rocks of theFrancis Lake Member, and then into the primary volcanic rocks and associated sedimentary rocks of theWeaver Lake and Echo Island Members mimics that of modern arcs in the Caribbean region (Larue et aL,1991). In the early stages of arc growth, subduction processes may have led to thermal expansion of theoverriding plate, resulting in minor compressional deformation and uplift of the Middle Triassic ocean floorrepresented by the Camp Cove Fonnation. Continued thermal uplift may have led to block faulting, resultingin deposition of coarse clastic deposits of the basal conglomerate by mass sediment gravity flows, and102suspected rapid subsidence and deposition of the basal Harrison Lake Formation (OADS I stage of Larue et al.,1991).Early explosive volcanic eruptions mark the imtiation of the volcanic sequence in early Late Toarciantime, and attest to shallow water depths or subareal volcanic exposure. Early pyroclastic activity is supersededby effusive volcanism as the arc stabilized (OADS II, Early Arc Growth). Epiclastic conglomerate, wooddebris, and sedimentaiy structures indicate that at least a portion of the arc was subaerial during this stage.The earliest stages of effusive volcanism was basaltic andesite to andesite in composition, superceded by thedacitic volcanism that characterizes the majority of the system. Isotopic and geochemical constraints suggestthe arc was a medium K calcalkaline system derived from a slightly enriched mantle wedge. The increase infelsic volcanism in the upper portions of the Weaver Lake Formation led to the emplacement of rhyoliticdomes, sills, dikes and associated quartz feldspar porphyry intrusions, and resulted in the increase inpyroclastic activity evident in the Echo Island Member (OADS III, Mature Stage). Volcanism waned in theEarly Bathoman, and was followed by approximately north-south directed compressional deformation of theformation. The Early Callovian Mysterious Creek Formation may represent an overlap sequence supplied withdetritus by the extinct arc (OADS IV of Larue et al., 1991).4.13 CONCLUSIONSIt has long been recognized that the Harrison Lake Formation constitutes a Middle Jurassic volcanicarc assemblage (Crickmay, 1925; Thompson, 1972; Pearson, 1973; Arthur, 1987; Arthur et al., 1993), butdetails about the evolution of the arc system have been lacking. New data presented herein serve to constrainthe stratigraphy, age, geochemistry, isotopic systematics and depositional history of the Harrison LakeFormation. Important new constraints include:1. New biostratigraphic data from tuffaceous strata of the Francis Lake Member indicate volcanism inthe Harrison Lake Formation began in the late Early Toarcian.1032. New U-Pb geochronology from a rhyolitic dome in the upper portion of the Weaver Lake Memberindicate that late, rhyolitic volcanism was active at 166 Ma (earliest Bathonian). Isotopic dates from the upperWeaver Lake Member overlap with the ages of both synvolcanic intrusions within the formation and largebatholithic complexes that intrude the formation on the west (Fig. 4.2).3. Stratigraphic constraints demonstrate the eruptive style of the volcanic arc progressed fromexplosive (early Late Toarcian to mid-Aalenian) to dominantly effusive (mid-Aalenian to Late Bajocian) andback to explosive (Late Bajocian to Bathonian?).4. Geochemical data indicate that the volcanic arc is of medium to high K calcalkaline affinity, isLREE enriched (LafYb = 1.5-4.25), and displays overall enrichment of LFSE relative to 1{FSE, supporting asubduction-related origin. Incompatible-compatible element ratios suggest a simple fractionation trend.5. Nd and Sr isotopic values for volcanic rocks demonstrate the relatively juvenile nature of themagmatic system, and support magmatic derivation from a slightly enriched mantle wedge.6. Stratigraphic fluctuations in ENd values suggest a temporal control on provenance variations, andindicate two component mixing between volcanic arc-derived material and more evolved (continental?)detritus.7. Mineralization in the Seneca area is associated with felsic volcanism, and is inferred to bestratigraphically associated with the Weaver LakefEcho Island member contact. This stratigraphic assessmentsuggests mineralization is probably Bajocian in age.8. Regional mapping in the Harrison Lake area suggests the Harrison Lake Formation containsmesoscopic folds and overturned strata that are absent in overlying strata. This structural contrast provides104evidence of post-Early Bathonian, pre-Callovian deformation in the southwestern Coast Belt, consistent withthe observations of Arthur (1987).These observations constrain the timing and geologic evolution of a Middle Jurassic volcanic arc inthe southern Coast Belt, and provide a basis for comparison with coeval volcanic sequences on adjacentterranes.105CHAPTER 5EARLY TO MIDDLE JURASSIC VOLCANISM ON WRANGELLIA:EVOLUTION OF THE BONANZA-HARRISON ARC SYSTEM1065. EARLY TO MIDDLE JURASSIC VOLCAMSM ON WRANGELLIA: EVOLUTION OF THEBONANZA-HARRISON ARC SYSTEM5.1 INTRODUCTIONSeveral terranes in the southern Canadian Cordillera contain Early to Middle Jurassic volcanic arcassemblages or associated volcaniclastic sedimentaiy sequences overlying Triassic basement. Correlationsbetween the volcanic assemblages are difficult due to complex internal stratigraphies, differences inunderlying Triassic basement, and a lack of detail concerning the evolution of individual arc assemblages.Documentation of the geologic evolution of each assemblage, particularly its geochemical and isotopiccharacteristics, is necessaiy prior to evaluating potential correlations.Wrangellia is the westernmost terrane in the southern Canadian Cordillera, and is separated from thenearest terrane to the east, the Harrison terrane, by Jurassic and Cretaceous plutons of the Coast PlutonicComplex (Fig. 5.1). Wrangellia and Harrison terranes both contain Lower to Middle Jurassic volcanic arcassemblages, but differences in the tinting of volcanism and the character of Triassic basement led to separateterrane designations (Monger et al., 1982). Lower to Middle Jurassic stratigraphic linkages between theBonanza Group and the Bowen Island Group of Wrangellia with the Harrison Lake Formation of the Harrisonterrane based on overlapping age ranges have been suggested (Roddick, 1965; Friedman et a!., 1990).However, supporting stratigraphic, geochemical and isotopic data have been lacking, and the separate terranedesignations remain (Monger and Journeay, 1994). The terranes are linked by bridging Middle Jurassic (<167Ma; Bathonian) plutons (Friedman and Armstrong, 1994).This chapter describes the stratigraphy, geochemistry, and isotopic signatures of Lower to MiddleJurassic volcanic arc assemblages of the Bonanza Group, Bowen Island Group and Harrison Lake Formation.Evaluation of the geologic, geochemical, and isotopic characteristics of these units is used to jonstrain sourcecharacteristics and the evolution of the Early to Middle Jurassic Bonanza-Harrison volcanic arc. This107801xiduioornonJSO3joUOfl!SOdTONsuuiiuosuijpui!1Ju1Murnqtrq‘1jTp1o3upiujuiqinosqijoduuiuiwjpffldw!S-ic1AJP!flJPHMPDUOS!.LIHIIPUJMHSVAJIII xaldwojUllP3SJC1OIplIO ag)SOaqJo SU!PSJWpapqpim xIdwo3nuopisojsapEaswj;stpojosuwaJB!SS3!UPIPS31aaJ3atpj!TpusnrJ/AIAAAA——I•vvvVVVVVV\vvvvvvvvv‘‘“.VVVVVVVVVVVVVV\\VVVVVVVVVVVVVVV.vvvvvvvvvVvvvVVVVVVVVVVVVVVVV‘Ivvvv.vvvvvvvvvv(•;\\jUOSI.L.IUH ‘VVVVVVVVvvVvvvVVVVVvvvVV’VVVVVVVVVVVVVVVVVVVVvvVvVvvvVvVVVVVVVVVVVVVvVVVVVVVVVv)VVVVVVVVVVVj-vvvvvkiVVVVvVvvVVVVVVVVVvvvV,VVVV,vvvvHk-“ VVV//\/3 VVVV4/////vVVV,%///////I, VVVVVVV‘VVVVV4////,///-VvVV%’,<VVVV///////7VVVVV1?///, VVVVV‘vvvvv\////, VVVVV‘VVVVVV’////, VVVVVV,VVVVS/VVI///7////7////7/investigation is the first integrated analysis of the geology, geochemistiy, and isotopic characteristics of theBonanza Group, Bowen Island Group, and the Harrison Lake Formation.5.2 GEOLOGIC SEllINGLower to Middle Jurassic strata of the Wrangellia and Harrison terranes form an arcuate band thatwraps around the southern end of the Coast Plutomc Complex. The eastern margin of Wrangellia is intrudedby Middle to Late Jurassic plutons of the western Coast Plutonic Complex (Nelson, 1979; Monger, 199 la).The southeastern portion of Wrangellia has been imbncated by west vergent compressional deformationassociated with both the Late Cretaceous San Juan Thrust System (Brandon et al., 1988) and the TertiaryCowichan Fold and Thrust system (England and Calon, 1991). Middle Jurassic and, to a lesser extent,Cretaceous plutons incorporate Bowen Island Group strata as roof pendants on the southwestern margin of theCoast Plutonic Complex (Friedman and Armstrong, 1994). Strata of the Bowen Island Group display tight toisoclinal folds and are strongly foliated.The Harrison terrane is exposed at the southeastern end of the Coast Plutonic Complex (Fig. 5.2).The western margin of the Harrison terrane is intruded by Middle Jurassic plutons (ca. 164-167 Ma); theeastern margin is the Harrison Lake shear zone, a ductile transcurrent fault separating subgreenschist faciesrocks of the Harrison terrane from amphibolite facies rocks to the east contained within the imbricate zone ofthe Coast Belt Thrust System (Journeay and Friedman, 1993). The Coast Belt Thrust System is a west vergentcontractional system formed along the eastern edge of the Coast Plutonic Complex in early Late Cretaceoustime. Frontal thrust faults at the leading edge of this system displace Jurassic and Early Cretaceoussupracrustal rocks and plutonic suites, including Lower to Middle Jurassic volcanic strata of the Harrisonterrane (Fig. 5.2). To the east of Harrison Lake and structurally above the Harrison terrane are high-grademetamorphic thrust nappes interleaved within the imbricate zone of the Coast Belt Thrust System (Journeayand Friedman, 1993). These thrust nappes represent metamorphosed supracrustal assemblages derived fromthe Harrison terrane and terranes to the east. Structurally overlying high grade metamorphic rocks but still10901=-.,---rI —=00‘I— herFigure 5.2 - Generalized geologic map of southwestern British Columbia, emphasizing Early to MiddleJurassic volcanic assemblages and Middle to Late Jurassic plutomc assemblages. Study areas areoutlined.rH r1’ILcqb_I-tC C—,0I00CI—00ccçD-.110within the central Coast Belt Thrust System are Lower and Middle Jurassic volcanic rocks and associatedsedimentaiy rocks correlated with the Harrison terrane (Journeay and Mahoney, 1994; Fig. 5.2). These rocksrepresent the eastemmost exposures of the Harrison terrane.5.3 TERRANE STRATIGRAPHYWrangellia is the most distinctive component of the morphogeologic Insular Belt, and extends fromthe southern end of Vancouver Island north to southeastern Alaska (Fig. 5.1). The terrane is remarkablyhomogeneous along its length; stratigraphic discussions herein will be limited to rocks exposed on VancouverIsland (Fig. 5.2, 5.3). The terrane consists of arc volcanic rocks and associated sedimentary rocks of theDevonian to Middle Pennsylvanian Sicker Group overlain by crinoidal limestone of the Middle Pennsylvanianto Permian Buttle Lake Group (Muller et al., 1981; Brandon et al., 1986; Monger and Journeay, 1992, 1994;Fig. 5.3). Paleozoic rocks are unconformably overlain by the Middle to Upper Triassic Vancouver Group,which contains a basal argillite overlain by the Kannutsen Fonnation, a thick sequence of tholeiitic basalt thatforms the most distinctive component of Wrangellia (Barker et al., 1989). The Karmutsen Formation isgradationally overlain by bioclastic limestone of the Upper Triassic Quatsino Formation, which in turn gradesupward into shale, limy mudstone, and tuffaceous wacke and conglomerate of the Upper Triassic Parsons BayFormation at the top of the Vancouver Group (Muller et al., 1981; Nixon et al., 1993, 1994).Arc volcanic rocks of the Lower Jurassic Bonanza Group gradationally overlie the Parsons BayFormation (Muller et al., 1974, 1981; Jeletsky, 1976; Nixon et al., 1993, 1994). The Bonanza Group isinterpreted to be cogenetic with Lower to Middle Jurassic plutonic rocks of the Island Intrusions andassociated metamorphic and migmatitic rocks of the Westcoast Complex (Debari and Mortensen, 1994). TheBonanza Group is locally unconformably overlain by Middle to Upper Jurassic shallow marine coarse clasticstrata of the Kyoquot Group (Muller et al., 1981; Yorath, 1991). Throughout the majority of the outcrop belt,an angular unconformity separates Lower Jurassic strata of the Bonanza Group from coarse sandstone and111Wrangellia Bowen Island Area HarrisonAlbianAptianLongarm Gambier GambierBanemian Formation Group GroupHauterManVakuiginianBerrisian7lthonian_______1 Ihook k Fm.LDh1PllIffi Mysterious Ck FmCallovian Kyoquot GroupBathonlanIF?Aalenian Harrison LakeToanianBonanza Bowen Island FormationIlensbachian IF?Group GroupSinemuIF IFHettanglan ——— — — — _2. —. .Norian Parsons Bay Parsons BayFormaton FormationCarnian Quatsino Formaton Quatsino FormationCamp Cove FmFigure 5.3- Simplified stratigraphic columns for Wrangellia and Harrison terranes and the Bowen Island area.Symbols: F = biostratigraphic control; Z = U-Pb age.112conglomerate of the Lower Cretaceous Longarm Formation and overlying Queen Charlotte Group (Muller etal., 1981; Yorath, 1991; Nixon Ct al., 1993, 1994).The Bowen Island Group is exposed in a series of roof pendants within the western margin of theCoast Plutonic Complex along coastal mainland British Columbia (Roddick and Woodsworth, 1979; Journeayand Monger, 1994; Fig. 5.2). The Bowen Island Group was named by Armstrong (1953) for a thicksuccession of altered volcanic rocks on Bowen Island. Subsequent mapping on the Sechelt Peninsula hasdocumented a stratigraphic contact between the Bowen Island Group and underlying Kannutsen and QuatsinoFormation (Monger, 1991, 1993; Journeay and Monger, 1994). Although the name Bowen Island Groupremains in the literature (Journeay and Monger, 1994) and will be applied herein to altered voLcanic rocksalong coastal British Columbia, the stratigraphic relationship between rocks mapped as Bowen Island Groupand underlying rocks of the Vancouver Group demonstrates that the Bowen Island Group is directly correlativewith the Bonanza Group strata and is therefore part of Wrangellia.The Bowen Island Group is unconformably overlain by granitoid-bearing conglomerate and volcanicrocks of the Lower Cretaceous Gambler Group (Roddick, 1965; Roddick and Woodsworth, 1979; Lynch, 1991;Journeay and Monger, 1994).The Harrison terrane consists of oceanic greenstone, chert, and greywacke of the Middle TriassicCamp Cove Formation unconformably overlain by arc volcanic rocks and associated sedimentary rocks of theLower to Middle Jurassic Harrison Lake Formation (Fig. 5.2; Chapter 4). Shallow marine sandstone and shaleof the Middle Jurassic Mysterious Creek Formation unconformably overlie the Harrison Lake Formation, andare in turn overlain by volcamclastic strata of the Upper Jurassic Bilihook Creek Formation (Monger, 1985;Arthur, 1987; Arthur et al., 1993). Granitoid-beanng polymict conglomerate of the Lower CretaceousPeninsula Formation unconformably overlies Jurassic rocks, and is gradationally overlain by volcanic rocks ofthe Lower Cretaceous Brokenback Hill Formation (Monger, 1985, Arthur, 1987). These latter two formationsare incorporated within the Gambier Group (Journeay and Monger, 1992, 1994)1135.4 LOWER TO MIDDLE JURASSIC VOLCANIC STRATAThe Bonanza Group, Bowen Island Group, and Harrison Lake Formation comprise Lower to MiddleJurassic volcanic arc assemblages and associated volcaniclastic sedimentary rocks. The complicated nature ofvolcanic facies relationships in arc systems leads to a lack of coherent stratigraphy and an absence ofregionally consistent marker beds, making traditional stratigraphic correlations difficult (Cas and Wright,1987). Middle to Lower Jurassic strata on the Wrangellia and Harrison terranes lack regionally consistentstratigraphy, and are characterized by rapid lateral and vertical facies changes, typical of a volcanic arc origin.The Bonanza Group comprises basalt, basaltic andesite, andesite, dacite, and rhyolite flows, breccia,tuff breccia and lapilli tuff intercalated with primarily thin to medium bedded tuff conglomerate, sandstone,siltstone, and argillite (Jeletsky, 1976, Muller Ct al., 1974, 1981; Nixon et al., 1993, 1994). Fragmental rocksare regionally more abundant than lava flows (Jeletsky, 1976). The base of the Bonanza Group consists of acoarsening upward succession of thin to thick bedded pyroclastic and epiclastic rocks gradationally overlyingthin to medium bedded limestone and clastic rocks of the Parsons Bay Formation (Jeletsky, 1976; Muller et al.,1981; Nixon et al., 1993, 1994). Fragmental rocks at the base of the group grade upward into the massive lavaflows and breccias that dominate the unit. Nixon et al. (1993) describe a bimodal assemblage of basaltinterdigitated with rhyolite on a scale of 100’s of metres near the base of the formation on northern VancouverIsland, and suggest flows become intermediate in composition to the east. Descriptions by Muller et al. (1974,1981) and Jeletsky (1976) indicate extreme lateral and vertical variability in composition and rock type in theBonanza Group throughout Vancouver Island. Volcanic rocks of the Bonanza Group are locally overlain byup to 600 m of volcaniclastic sedimentary rocks, including conglomerate, tuffaceous sandstone, siltstone, andargillite (Jeletsky, 1976; Muller et al., 1981).The Bonanza Group includes tuffaceous shale, siltstone, and sandstone of the Lower Jurassic(Sinemurian) Harbledown Formation. The Harbledown Formation is interpreted to be an eastern volcaniclastic114facies distal to the western volcanic edifices represented by the majority of the lower Bonanza Formation(Tipper etal., 1991).Metamorphic overprint and limited exposure of the Bowen Island Group preclude detailedstratigraphy. The Bowen Island Group is metamorphosed to lower greenschist(?) grade, and consists ofmassive andesitic flows, tuffbreccias and lapilli tuff interbedded with thin bedded tuff tuffaceous sandstone,siltstone and argillite (Roddick, 1965, Friedman et aL, 1990; Monger, 1992). The base of the Bowen IslandGroup has not been described, but it is mapped in stratigraphic continuity with metabasalt of the KarmutsenFormation and marble of the Quatsino Formation in the northwestern end of the outcrop belt (Monger, 1993;Journeay and Monger, 1994). Thin-bedded tuff and volcarnclastic sedimentaiy rocks dominate thenorthwestern end of the outcrop belt, and massive volcanic flows dominate southeastern exposures (Friedmanet al., 1990). The Bowen Island Group is apparently unconformably overlain by granitoid bearingconglomerate of the Lower Cretaceous Gambier Group (Roddick and Woodsworth, 1979; Joumeay andMonger, 1994), although this contact is commonly faulted (Monger, 1993).The Harrison Lake Formation consists of basaltic andesite to rhyolite flows, breccias, tuffbreccias,lapilli tuff and associated volcaniclastic sedimentaiy rocks. The fonnation is subdivided into four members,including, in ascending order, a basal conglomerate (Celia Cove Member), argillaceous interval (Francis LakeMember), primary volcanic rock unit (Weaver Lake Member), and tuffaceous sedimentary rock unit (EchoIsland Member; Arthur, 1987, Arthur et at., 1993; Mahoney et al., 1994; Chapter 4). The base of theformation is an unconformity above oceanic rocks of the Middle Triassic Camp Cove Formation. The majorityof the formation consists of interdigitated volcanic flows, breccias, tuffbreccias, lahanc breccias, hyaloclastite,and intermediate dikes and sills. Mahoney et al. (1994; Chapter 4) recognize a compositional transition fromandesitic basalt and andesite near the base of the formation to rhyolite near the top of the fonuation. Theupper part of the primary volcanic member is locally characterized by rhyolite domes and associated dikes andsills. The upper tuffaceous unit consists of 400-600 m of thin to thick bedded conglomerate, sandstone,siltstone, and argillite containing abundant resedimented pyroclastic debris.1155.5 AGE CONSTRAINTSAge constraints in the Bonanza Group, Bowen Island Group, and Harrison Lake Formation isprovided by rare marine fossils and radiometric dates in the Bowen Island Group and Harrison LakeFormation.The Bonanza Group is biostratigraphically constrained to be early Sinemurian to Bajocian in age(Poulton, 1980; Muller et al., 1981). The majority of fossils identified from the Group are ainmonites andbivalves of early Sinemurian to late Pliensbachian age recovered from sedimentary beds intercalated withvolcanic rocks. However, Tngoniid bivalves, Myophorella taylori, of early Bajocian age were recovered in thelate 1970’s from sedimentary interbeds within volcanic strata of the Bonanza Group exposed in the mine pit atthe Island Copper Mine, northern Vancouver Island (Poulton, 1980). This fossil locality and age assignmentwere largely ignored by subsequent workers (e.g. Muller et al., 1981; Nixon et al., 1993, 1994). The overlapbetween this biostratigraphic age and the U-Pb ages of the coeval Island Intrusions strongly indicate volcanismin the Bonanza Group was active until at least Early Bajocian time.The biostratigraphic age determinations agree well with the radiometric ages determined forcomagmatic intrusions of the Island Intrusions and Westcoast Complex (DeBari and Mortensen, 1994). Agesfor the Island Intrusions and Westcoast Complex overlap, and range from 190- 176 Ma (Pliensbachian toAalenian).The age of the Bowen Island Group is constrained to be Sinemurian to Toarcian by biostratigraphyand one radiometric age determination. Probable Sinemurian ainmonites have been recovered from twolocalities in the northwestern portion of the outcrop belt (Friedman et aL, 1990; Monger, 1991a). Zirconrecovered from a flow-banded rhyolite yielded a U-Pb age of 185 +81-3 Ma (Toarcian; Friedman et al., 1990).116The age of the Harrison Lake Formation is constrained to be Pliensbachian(?) to early Bathonian bybiostratigraphy and one radiometric age determination. The lower conglomerate member of the Harrison LakeFormation is undated, but contains Middle Triassic chert clasts derived from the underlying Camp CoveFormation (Arthur, 1987). The conglomerate is gradationally overlain by fine-grained clastic rocks of theFrancis Lake Member, which yields ammonites of late Early Toarcian age. The gradational contact with theoverlying Francis Lake Member suggests the conglomerate and overlying fine-grained rocks were deposited inthe same depositional system, and are therefore nearly age equivalent. The most probable age of the basalconglomerate is late Pliensbachian to Early Toarcian. Tuff beds intercalated with marine argillite constrainthe initiation of volcanism in the Harrison Lake Formation to be late Early Toarcian (Chapter 4). Zirconrecovered from a rhyolite dome complex near the top of the Weaver Lake Member yielded a U-Pb age of 166+1- 0.4 Ma, suggesting the upper part of the formation is early Bathonian in age. The minimum age of the topof the Harrison Lake Formation is constrained by overlying strata to be early Callovian.5.6 GEOCHEMISTRYAU volcanic arc assemblages tend to display similar geochemical signatures indicative of theirsubduction-related tectonic setting, but identification of individual volcanic arc assemblages is inherentlydifficult. The absence of site-specific geochemical markers in typical island arc suites make it difficult todistinguish between potentially unrelated volcanic assemblages in complex tectonic settings such as theCanadian Cordillera. The most profitable avenue of correlation between tectonically disrupted volcanic suitesis identification of consistent, systematic geochemical trends that suggest a genetic relationship between suites.Identification of such geochemical trends coupled with temporal and isotopic constraints are the best methodscurrently available to relate disrupted suites.Geochemical analyses are presented from representative suites of the Bonanza Group, Bowen IslandGroup and Harrison Lake Formation. Samples in the Bonanza Group constitute two subgroups: the first wascollected in the Nootka Sound region of northern Vancouver Island (n=23), and the second was collected from117Trace ElementsNiCrVBaRbSrZrYNbCuZnPbLaCePrNdSmEuGdThDyHoErTmYbLuHfThUTaCs5 22 -3 27-3 50 3 12928 268 106 3201069 484 769 -66 51 24 -303 658 522 407234.00 78.76 138.10 35.732;.20 11.10 16.0050.20 27.50 36.20 -23.50 14.30 23.206.06 3.78 5.98 -1.39 1.24 1.71 -0.97 0.58 0.964.32 2.11 3.350.67 0.32 0.49 -5.93 2.18 3.62 -4.60 1.90 2.00 -2.10 0.81 0.99 -0.69 0.26 0.52 -0.77 1.41 0.67Ce/YbBa/LaLaIFh1NbBa/NbZr/YNormalized valuesLa/Yb 3.4La/Sm 2.23.5 3.11.8 1.66.3 10.79.1 38.412.1 3.31.1 1.811.7 16.0 11.420.5 25.8 33.27.0 7.2 7.02.2 2.1 2.345.0 53.0 76.34.3 5.0 3.64.0 5.3 3.71.9 2.5 2.0Table 5. la- Geochemical data from the Bonanza Group in the Port Alberni study area.Bonanza Group - Alberni areaSample 91-7 91-9a 91-9b 91-16 91-77 91-85 91-97 93-412B 93-414A 93-416ASi02 70.00 52.19 58.12 55.15 51.62 49.58 53.80 47.82 55.53 55.53A1203 15.03 18.06 17.98 18.02 18.19 16.56 17.76 17.04 21.27 17.77Ti02 0.57 0.94 1.07 1.01 0.94 0.92 0.91 1.12 0.64 0.99FeO* 2.91 9.00 6.99 6.68 9.32 8.40 8.22 9.90 5.79 8.65MnO 0.09 0.16 0.15 0.16 0.21 0.16 0.19 0.20 0.19 0.18CaO 1.46 9.25 6.77 8.01 8.52 13.69 6.64 11.65 7.15 8.62MgO 0.83 5.24 2.23 3.73 5.78 7.80 5.06 4.95 3.13 4.02K20 3.30 1.51 1.43 1.47 1.52 0.23 1.16 0.37 1.09 1.26Na20 5.38 2.44 3.89 3.16 2.55 1.90 5.12 3.22 3.40 2.88P205 0.13 0.27 0.60 0.44 0.29 0.19 0.30 0.26 0.45 0.27Total 100.03 100.05 99.99 100.02 99.95 100.33 100.04 96.52 98.64 100.17LOt38 75 4181 309 104259 305 221• 43 492- 2 30660 690 52068.59 42.73 83.46- 18.01 -- 1.54 -10 50 25 55 46 6 2164 89 76 76 125 41 96• 1.X8- 4.73 12.80- 10.40 27.50• 1.71 -- 8.62 17.90- 2.57 4.29- 1.09 1.32- 2.62 -- 0.49 0.57- 3.16 -- 0.65 -• 1.91 -- 0.26• 1.66 2.57- 0.26 0.33- 1.08 2.48- 0.39 1.90- 0.27 <2.8- 0.14 0.31- 0.11 0.981344359241460386.0020.095.3655894.3811.7322.592.9313.153.731.253.580.633.910.772.250.321.930.321.681.680.560.290.2869155024575128.0025.5610.37131013.9021.2942.175.1121.495.131.654.500.764.600.972.810.412.630.422.762.970.810.501.422023750619347106.0029.726.6371864.1015.2530.893.8016.454.701.374.600.845. 13.0 10.848.2 43.6 48.14.8 5.8 8.0118Bonanza Group - Nootka areaSample 92-145a 92-153A 92-169ar 92-170a 92-177 92-185 92-187 92-189 92-230a 92-235 92-241Si02 48.36 47.74 48.74 49.23 52.42 56.56 55.04 53.74 57.10 62.94 57.06A1203 15.11 19.18 17.34 15.67 18.49 15.11 16.30 14.93 17.01 15.29 15.67Ti02 0.81 0.50 1.00 2.20 1.01 1.99 1.30 1.73 1.22 1.23 1.40Fe0 10.37 10.64 11.13 12.42 7.99 9.77 8.70 12.01 8.66 6.77 9.09MnO 0.20 0.19 0.17 0.21 0.32 0.33 0.26 0.22 0.15 0.21 0.23CaO 10.75 10.72 7.10 7.42 7.49 4.91 4.38 6.74 7.03 2.78 4.60MgO 11.36 8.08 8.02 6.46 5.96 2.76 5.05 4.46 3.28 2.00 3.051(20 0.46 0.14 1.16 1.37 2.34 2.93 2.87 0.73 1.13 2.79 2.19Na20 1.74 1.87 3.67 3.69 3.58 4.10 4.80 3.80 3.50 5.11 5.40P205 0.14 0.04 0.17 0.37 0.20 0.58 0.19 0.27 0.28 0.43 0.33Total 100.45 100.28 99.73 100.41 100.67 100.11 99.86 99.95 100.31 100.29 100.01WITrace ElementsNi 190 192 78 61 55 3 48 30 5 9 7Cr 543 81 47 62 159 12 67 18 5 7 9V 267 201 290 295 247 149 215 437 264 62 270Ba 200 34 443 300 1591 866 714 163 451 660 507Rb 8 4 20 31 60 69 62 14 28 62 52Sr 169 110 277 229 393 83 136 229 414 163 172Zr 47.61 18.46 56.43 213.60 87.64 188.48 237.20 137.59 84.65 313.17 182.44y 19.91 20.98 20.13 55.21 22.91 60.81 51.08 43.80 22.64 65.46 38.77NI, 1.96 0.23 1.96 4.91 3.01 4.96 4.84 3.05 3.35 8.06 6.15Cu 39 88 80 87 62 5 87 119 32 7 48Zn 106 46 154 135 141 219 200 103 91 94 121Pb 0.88 0.76 1.88 2.50 8.59 2.08 23.18 1.02 1.83 3.27 5.17La 4.00 0.79 3.88 11.27 7.61 12.87 10.98 8.50 8.98 18.43 13.65Ce 9.40 1.78 8.99 26.80 16.39 31.88 26.71 20.88 19.27 41.71 28.87Pr 1.43 0.35 1.37 3.91 2.24 4.76 3.88 3.07 2.63 5.82 3.90Nd 7.24 2.16 7.13 20.45 10.67 25.40 19.76 15.63 12.98 28.35 18.60Sm 2.47 1.20 2.44 6.87 3.11 8.27 6.25 5.39 3.78 8.60 5.40Eu 0.94 0.58 0.97 2.14 1.13 3.10 1.66 1.66 1.33 2.53 1.40Gd 2.85 2.01 2.84 7.78 3.5L 9.58 6.97 6.04 3.83 9.46 5.85Th 0.54 0.46 0.55 1.47 0.65 1.80 1.34 1.17 0.69 1.79 1.08Dy 3.59 3.31 3.54 9.44 4.19 11.34 8.78 7.54 4.07 11.40 7.02Ho 0.73 0.77 0.74 1.99 0.86 2.33 1.86 1.61 0.83 2.40 1.47Er 2.12 2.48 2.21 5.81 2.47 6.80 5.71 4.70 2.47 7.01 4.43Tm 0.29 0.34 0.29 0.78 0.34 0.91 0.78 0.65 0.33 0.98 0.60Yb 1.82 2.35 1.76 5.03 2.11 5.71 5.05 4.06 2.14 6.13 3.80Lu 0.29 0.39 0.29 0.81 0.34 0.89 0.78 0.63 0.34 0.98 0.60HI’ 1.34 0.63 1.21 4.85 2.07 4.57 5.66 3.30 2.14 6.97 4.14Th 0.46 0.08 0.27 1.10 0.68 1.53 1.51 0.79 1.03 2.70 1.72U 0.17 0.02 0.11 0.55 0.27 0.58 0.60 0.33 0.40 1.01 0.66Ta 0.12 0.02 0.12 0.36 0.18 0.31 0.31 0.19 0.20 0.51 0.39Cs 0.38 0.22 0.46 0.36 1.78 0.44 1.01 0.62 0.87 0.45 0.32Ce/Yb 5.2 0.8 5.1 5.3 7.8 5.6 5.3 5.1 9.0 6.8 7.6Ba/La 50.0 43.0 114.2 26.6 209.1 67.3 65.0 19.2 50.2 35.8 37.1La/Th 8.7 9.9 14.4 10.2 11.2 8.4 7.3 10.8 8.7 6.8 7.9LaJNb 2.0 3.4 2.0 2.3 2.5 2.6 2.3 2.8 2.7 2.3 2.2BaJNb 102.0 147.8 226.0 61.1 528.6 174.6 147.5 53.4 134.6 81.9 82.4Zr/Y 2.4 0.9 2.8 3.9 3.8 3.1 4.6 3.1 3.7 4.8 4.7Normalized valuesLa/Yb 1.4 0.2 1.5 1.5 2.4 1.5 1.4 1.4 2.8 2.0 2.4La/Sm 1.0 0.4 1.0 1.0 1.5 0.9 1.1 1.0 1.4 1.3 1.5Table 5. lb- Geochemical data from the Bonanza Group in the Nootka Sound study area.119Si02A1203Ti02FeO*MnOCaOMgOK20Na20P205Total54.11 48.84 49.68 57.60 54.2114.74 17.25 18.19 17.43 17.481.85 0.98 1.95 1.07 1.6412.75 9.89 11.20 8.44 9.680.24 0.25 0.18 0.13 0.136.13 11.39 8.53 2.91 4.924.29 7.75 4.74 3.20 3.710.95 1.13 1.42 1.92 0.273.46 1.83 2.63 7.18 6.650.32 0.15 0.33 0.04 0.47100.23 100.54 100.07 100.86 100.2255.77 56.99 59.44 54.3316.48 17.45 16.34 21.241.46 0.87 0.90 1.039.73 6.94 7.46 7.180.18 0.11 0.23 0.184.72 6.36 3.71 3.434.04 4.44 3.06 3.981.53 2.72 2.63 0.845.61 3.40 5.56 6.600.29 0.21 0.20 0.26100.90 100.24 100.35 99.8557.35 52.95 52.6717.75 19.08 16.860.88 0.98 1.397.27 8.97 9.860.15 0.16 0.237.04 8.36 5.793.93 4.09 5.052.03 1.62 1.462.62 2.81 5.500.22 0.22 0.27100.03 100.23 99.08Trace ElementsNi 15Cr 6V 441Ba 364Rb 26Sr 244Zr 127.37Y 42.34Nb 3.86Cu 18Zn 130Pb 2.05La 9.81Ce 22.03Pr 3.19Nd 16.33Sm 5.35Eu 1.78Gd 6.12Th 1.17Dy 7.44Ho 1.57Er 4.82Tm 0.65Yb 4.08Lu 0.64Hf 3.33Th 1.09U 0.50Ta 0.24Cs 0.35Table 5. lb (cont.) - Geochemical data from the Bonanza Group in the Nootka Sound study area.Bonanza Group - Nootka areaSample 92-244 92-249a 92252 92-253 92-256 92-258R 92-259 92-261 92-265b 92-270a 92-272 93-329LOT82 39 28 -3210 54 38 -3306 303 146 216215 270 7429 24 8265 274 137 11342.89 163.46 142.11 158.8118.16 40.47 43.762.02 5.03 5.0254 168 3 11108 115 61 820.97 2.96 1.644.43 10.58 8.799.76 24.39 21.581.41 3.50 3.177.10 17.54 16.752.35 5.42 5.610.97 1.82 1.752.79 6.08 6.390.53 1.16 1.193.32 7.25 7.930.69 1.54 1.702.04 4.58 5.160.27 0.61 0.711.73 3.87 4.500.27 0.60 0.731.23 3.63 3.880.44 1.15 1.620.18 0.41 0.630.13 0.36 0.32O.77 0.55 0.3318 30 1121 80 17303 197 212393 514 51031 61 51272 284 92138.86 104.43 202.3537.94 23.77 39.603.68 4.41 4.8713 9 49118 62 1262.10 2.59 1.789.04 16.16 12.2920.38 30.73 27.512.90 3.80 3.6414.40 16.91 16.814.67 4.36 5.081.42 1.28 1.345.26 4.40 5.521.04 0.72 1.036.59 4.11 6.751.40 0.83 1.444.29 2.36 4.420.59 0.30 0.613.67 2.00 4.030.58 0.32 0.653.20 2.68 4.751.19 2.34 2.040.47 0.92 0.810.23 0.32 0.330.39 0.76 0.455.6 15.4 6.843.5 31.8 41.57.6 6.9 6.02.5 3.7 2.5106.8 116.6 104.73.7 4.4 5.1286220225122447129.2922.034.96401022.578.7119.682.7913.343.821.243.680.684.150.832.350.312.080.342.982.861.080.410.489.528.83.01.850.65.921 17 3153 29 74191 246 230550 453 68144 34 23393 395 27084.82 64.54 128.0020.73 19.51 33.813.44 2.70 4.2656 10 4872 78 1382.17 2.54 7.8210.32 8.06 9.1119.09 15.51 20.352.46 2.11 2.8211.15 9.62 13.783.00 2.78 4.300.99 1.00 1.653.14 3.00 5.160.56 0.57 0.963.55 3.53 6.000.74 0.75 1.222.13 2.12 3.680.30 0.29 0.511.95 1.87 3.120.33 0.31 0.511.90 1.59 3.021.86 1.19 1.070.69 0.47 0.420.33 0.24 0.250.73 0.92 0.229.8 8.3 6.553.3 56.2 74.85.5 6.8 8.53.0 3.0 2.1159.9 167.8 159.94.1 3.3 3.8Ce/Yb 5.4Ba/La 37.1La/Tb 9.0LaJNb 2.5Ba/Nb 94.3Zr/Y 3.0Normalized valuesLa/Yb 1.6La/Sm 1.15.6 6.348.5 25.510.1 9.22.2 2.1106.4 53.72.4 4.01.7 1.81.1 1.6 5.3 2.0 2.8 3.5 2.8 1.91.0 1.2 2.3 1.5 1.4 2.1 1.8 1.3120Harrison Lake FormationSample 043BM92 116JBM92 122JBM92 123JBM92 142JBM92 151JBM92 287JBM92 2911BM92 292JBM92S102 61.68 58.12 75.29 68.40 56.32 74.74 63.00 67.51 74.68A1203 16.76 19.16 12.83 15.73 19.23 13.47 0.67 15.58 14.36Ti02 0.87 0.77 0.29 0.51 0.83 0.32 15.90 0.51 0.25Fe0 7.30 6.98 1.50 4.18 9.56 2.96 5.13 4.19 2.15MnO 0.15 0.20 0.05 0.09 0.18 0.10 0.10 0.09 0.05CaO 5.03 2.37 1.51 1.61 3.16 0.63 0.63 1.20 5.12MgO 2.54 4.62 0.45 2.46 5.82 1.03 1.03 2.54 0.80K20 0.54 1.54 3.66 3.67 2.56 1.19 1.19 4.09 1.38Na20 4.56 5.95 3.49 3.69 1.75 5.27 5.27 4.03 0.82P205 0.25 0.21 0.07 0.17 0.22 0.07 0.07 0.15 0.05Total 100.49 100.68 99.29 100.96 100.67 100.09 96.65 100.36 99.90WITrace ElementsNi - 4 -- 9 •- S -Cr 10 14 15 13 25 17 16 17 10V 95 170 14 77 221 4 83 76 11Ba 185 658 1438 1189 424 395 838 1175 616Rb 7 29 37 54 61 13 33 52 30Sr 147 231 147 244 33 153 289 121 219Zr 68.12 128.34 135.92 146.53 105.54 149.39 131.10 153.98 124.63Y 30.40 23.60 19.14 23.26 21.72 56.20 33.57 24.85 19.01Nb 2.67 4.98 5.40 6.02 4.48 5.32 6.62 6.49 5.44Cu 19 7 8 34 9 26 2Zn 88 108 46 53 117 62 68 88 45Pb 2.32 3.40 1.37 4.18 1.92 1.58 3.91 17.55 7.73La 7.37 14.33 9.03 17.85 12.60 15.32 15.66 16.47 14.16Ce 15.73 27.75 18.72 28.71 24.31 29.73 30.56 27.24 24.98Pr 2.34 3.36 2.30 3.40 3.02 4.66 3.95 3.28 2.80Nd 11.81 14.92 9.99 13.86 13.34 21.95 17.82 13.66 10.93Sm 3.83 3.90 2.49 3.20 3.59 6.63 4.91 3.38 2.63Eu 1.43 1.15 0.69 0.98 1.02 1.69 1.35 0.93 0.65Gd 4.26 3.85 2.57 2.98 3.56 7.21 5.01 3.38 2.41Th 0.82 0.67 0.48 0.56 0.63 1.42 0.90 0.59 0.4Dy 5.45 4.21 2.96 3.64 4.04 9.27 5.87 3.92 3.01Ho 1.18 0.88 0.67 0.80 0.83 1.98 1.25 0.84 0.65Er 3.45 2.69 2.01 2.38 2.46 5.97 3.72 2.58 2.07Tm 0.48 0.37 0.30 0.35 0.34 0.83 0.53 0.36 0.32Yb 2.98 2.37 2.00 2.31 2.25 5.42 3.44 2.46 2.00Lu 0.49 0.40 0.36 0.40 0.38 0.92 0.57 0.44 0.36Hf 1.82 2.85 3.11 3.39 2.68 3.68 3.16 3.14 3.08Th 0.71 3.23 2.61 3.98 2.97 1.12 1.94 3.39 2.57U 0.25 0.98 0.83 1.31 0.96 0.44 0.66 1.10 1.00Ta 0.20 0.43 0.42 0.49 0.39 0.38 0.43 0.52 0.54Cs 0.36 0.18 0.48 0.38 0.94 0.10 1.21 0.15 2.50Ce/Yb 5.3 11.7 9.4 12.4 10.8 5.5 8.9 11.1 12.5BaJLa 25.1 45.9 159.2 66.6 33.7 25.8 53.5 71.3 43.5La/Th 10.4 4.4 3.5 4.5 4.2 13.7 8.1 4.9 5.5LaJNb 2.8 2.9 1.7 3.0 2.8 2.9 2.4 2.5 2.6Ba/Nb 69.3 132.1 266.3 197.5 94.6 74.2 126.6 181.0 113.2Zr/Y 2.2 5.4 7.1 6.3 4.9 2.7 3.9 6.2 6.6Nonnalized valuesLa/Yb 1.6 4.0 3.0 5.1 3.7 1.9 3.0 4.4 4.7La/Sm 1.2 2.2 2.2 3.4 2.1 1.4 1.9 3.0 3.3Table 5. ic - Geochemical data from the Harrison Lake Formation.121Harrison Lake FormationSample 293JBM92 306JBM92 3073BM92 13JBM93 14JBM93 15JBM93 17JBM93 213BM93 22JBM93 25JBM93Si02 73.44 79.57 68.11 75.52 77.39 67.44 70.40 68.11 77.88 74.53A1203 15.08 11.35 15.60 13.38 13.37 15.53 14.73 14.81 12.55 13.53Ti02 0.25 0.20 0.61 0.25 0.23 0.59 0.51 0.54 0.16 0.30FeO* 1.20 1.38 4.45 1.30 1.32 3.86 4.58 4.30 1.42 1.91MnO 0.04 0.03 0.06 0.03 0.04 0.08 0.17 0.12 0.05 0.05CaO 3.80 0.43 3.62 2.22 1.19 1.81 1.28 3.03 0.87 1.03MgO 0.68 0.45 1.77 0.72 0.57 1.10 1.14 1.35 0.35 0.76K20 3.27 2.91 1.62 0.24 0.25 3.89 0.07 1.49 1.63 3.33Na20 1.55 3.36 4.12 5.11 6.67 5.62 7.41 4.56 5.65 4.57P205 0.05 0.06 0.21 0.05 0.05 0.22 0.15 0.16 0.03 0.07Total 99.49 99.88 100.65 98.82 101.08 100.14 100.43 98.48 100.59 100.08WITrace ElementsNi - - 8 9 4 5 7 8 8Cr 13 16 12 3 5 4 0 2 3 4V 10 11 43 28 13 53 0 45 4 24Ba 1190 1060 154 239 91 937 71 667 940 929Rb 84 82 16 1 3 57 1 23 20 38Sr 163 113 72 242 200 161 123 339 147 152Zr 124.34 93.46 110.68 115.00 112.00 137.00 87.00 130.00 123.00 128.00y 19.40 13.98 31.08 21.01 13.94 36.08 36.85 31.61 23.22 21.24Nb 5.44 6.31 3.38 2.95 3.47 7.22 3.87 7.00 5.36 4.77Cu 6 6 13 4 5 8 8Zn 38 35 75 8 11 64 92 69 30 32Pb 8.07 1.55 2.53 0,84 0.69 6.50 1.42 4.51 2.32 2.27La 12.65 16.50 11.55 11.10 10.66 18.38 7.38 12.46 14.24 14.42Ce 24.62 28.13 22.86 22.24 19.99 31.50 17.06 24.93 26.51 23.89Pr 2.84 3.01 3.31 2.81 2.32 4.45 2.53 3.25 3.07 3.07Nd 11.59 11.15 15.41 11.77 9.01 19.59 12.52 15.16 12.74 12.27Sm 2.72 2.11 4.59 3.02 2.16 5.19 4.06 4.53 3.11 2.99Eu 0.62 0.57 1.46 0.68 0.57 1.37 1.39 1.25 0.69 0.77Gd 2.76 1.89 4.67 2.82 1.83 5.16 4.96 4.50 2.88 2.90Th 0.51 0.32 0.84 0.51 0.32 0.95 0.97 0.84 0.53 0.54Dy 3.25 2.07 5.48 3.30 2.14 5.98 6.43 5.35 3.68 3.25Ho 0.68 0.42 1.17 0.72 0.46 1.25 1.44 1.15 0.81 0.65Er 2.07 1.34 3.44 2.25 1.47 3.79 4.27 3.45 2.48 2.11Tm 0.29 0.21 0.48 0.33 0.22 0.51 0.60 0.49 0.37 0.32Yb 2.00 1.43 3.02 2.26 1.63 3.36 3.90 3.20 2.67 2.09Lu 0.34 0.27 0.47 0.40 0.28 0.54 0.65 0.53 0.49 0.35Hf 3.40 2.41 2.68 2.97 2.76 3.41 2.63 3.27 3.40 3.05Th 3.08 5.30 1.25 1.87 1.56 2.59 0.70 1.76 2.39 2.43U 1.13 1.77 0.43 0.70 0.52 0.96 0.26 0.60 0.91 0.83Ta 0.58 0.63 0.21 0.20 0.22 0.43 0.21 0.39 0.37 0.30Cs 1.73 0.34 1.18 0.07 0.05 0.12 0.01 0.83 0.64 0.25Ce/Yb 12.3 19.7 7.6 9.8 12.3 9.4 4.4 7.8 9.9 11.4Ba/La 94.1 64.2 13.3 21.5 8.5 51.0 9.6 53.5 66.0 64.4I.a/Th 4.1 3.1 9.2 5.9 6.8 7.1 10.5 7.1 6.0 5.9La/Nb 2.3 2.6 3.4 3.8 3.1 2.5 1.9 1.8 2.7 3.0Ba/Nb 218.8 168.0 45.6 81.0 26.2 129.8 18.3 95.3 175.4 194.8Zr/Y 6.4 6.7 3.6 5.5 8.0 3.8 2.4 4.1 5.3 6.0Nonnalized valuesLa/Yb 4.2 7.6 2.5 3.2 4.3 3.6 1.2 2.6 3.5 4.5LaJSm 2.8 4.8 1.5 2.2 3.0 2.2 1.1 1.7 2.8 2.9TableS. ic (cont.)-Geochemical data from the Harrison Lake Formation.122Bowen Island Group Middle Jurassic PlutonsSample 90JBM93 91JBM93 Bowen RMF-89-1 MV-88-37Si02 50.01 52.28 79.18 71.37 69.16A1203 16.97 18.07 11.77 14.58 14.54Ti02 0.74 0.78 0.05 0.40 0.49Fe0 9.06 9.18 1.68 3.48 5.13MnO 0.19 0.14 0.02 0.05 0.10CaO 10.79 7.58 0.36 2.71 5.67MgO 8.98 5.86 0.31 0.86 1.51K20 0.49 0.73 2.43 2.15 0.54Na20 1.81 5.28 5.06 5.38 3.28P205 0.20 0.13 0.01 0.12 0.10Total 99.25 100.02 100.87 101.10 100.52L01Trace ElementsNi 129 27 15 11 15Cr 430 38 8 13 18V 235 253 1 34 85Ba 133 528 584 873 323Rb 12 10 27 23 9Sr 354 640 67 218 307Zr 64.00 62.00 152.00 138.00 113.00Y 17.58 19.50 80.61 23.64 32.99Nb 2.69 1.86 29.54 6.94 3.67Cu 66 68 23 30 30Zn 73 78 46 13 42Pb 2.00 4.15 9.60 2.61 4.61La 7.68 5.05 21.84 15.10 11.47Ce 15.52 10.93 46.24 29.24 23.41Pr 2.21 1.64 5.87 3.38 3.25Nd 10.42 8.35 25.45 13.34 14.79Sm 2.93 2.43 7.34 3.25 4.30Eu 1.02 0.94 0.53 0.86 1.11Gd 2.96 2.86 9.06 3.25 4.42Th 0.51 0.53 1.89 0.60 0.82Dy 3.17 3.33 12.87 3.78 5.36Ho 0.64 0.67 2.79 0.80 1.14Er 1.92 1.97 8.39 2.55 3.48Tm 0.26 0.27 1.25 0.38 0.51Yb 1.61 1.73 8.12 2.53 3.29Lu 0.26 0.27 1.25 0.45 0.53Hf 1.24 1.08 6.21 3.19 2.89Th 0.63 0.39 8.72 2.85 1.58U 0.22 0.16 2.15 0.95 0.61Ta 0.15 0.11 2.28 0.47 0.20Cs 0.23 0.55 0.09 0.08 0.15Ce/Yb 9.6 6.3 5.7 11.6 7.1Ba/La 17.3 104.6 26.7 57.8 28.2La/Th 12.2 12.9 2.5 5.3 7.3La/Nb 2.9 2.7 0.7 2.2 3.1Ba/Nb 49.4 283.9 19.8 125.8 88.0Zr/Y 3.6 3.2 1.9 5.8 3.4Normalized valuesLa/Yb 3.1 1.9 1.8 3.9 2.3L.a/Sm 1.6 1.3 1.8 2.8 1.6TableS. id - Geochemical data from the Bowen Island Group and Middle Jurassic pluton.123the Port Albemi area in southern Vancouver Island (n=1O; Fig. 5.2). Bowen Island Group samples werecollected from Bowen Island itself and exposures to the north (n=3). Harrison Lake Formation samplesrepresent a unifonn coverage of the entire outcrop belt (n=19; Fig. 5.2). Representative samples of MiddleJurassic plutons west of the Harrison terrane are provided for comparative purposes (n=2; Table 5.1). Adescription of analytical techniques is given in Appendix A.A. Major and Trace Element GeochemistiyVolcanic rocks from all three units range from basaltic andesite and andesite to rhyolite. The lack ofmore felsic samples from the Bonanza Group (Fig. 5.4) is probably the result of sampling bias in thisinvestigation, as rhyolitic rocks are reported from throughout the Group (Muller et al., 1974; Jeletsky, 1976;Nixon et al., 1994). Samples from the Bonanza Group were collected in conjunction with a detailed petrologicanalysis of the Group, and collection was limited to rocks of basaltic to andesitic composition to facilitatepetrologic modeling of the initial source region. Major and trace element values for all three units indicate therocks are of tholeiitic to caic-alkaline magmatic affinity; the samples define a colinear tholeiitic to calcalkalinetrend on an AFM diagram (Fig. 5.4a). There is overlap between samples from all units, and the AFM trend issegmented, with Bonanza Group samples plotting near the tholeiitic/calcalkaline boundaiy, and Harrison LakeFormation rocks displaying a stronger calcalkaline affinity. Trace element abundances and trace elementratios, such as Ba/La, LaiTh, La/Nb, are characteristic of subduction-related volcanic suites (Gill, 1981).Linear to curvilinear patterns characterize plots of major and trace elements against Si02 (Fig. 5.5).The Nootka Sound samples from the Bonanza Group are divisible into two distinct subgroups: 1) incompatibleelement enriched basaltic andesite-andesite containing abnormally high Ti02,FeO*, Zr, Y, and HREE (Figs.5.5, 5.8b); and 2) basalt and basaltic andesite displaying incompatible element values within the range ofnormal tholeiitic to calcalkaline rocks (Gill, 1981). These two subgroups are well segregated on Ti02 and ZrHarker diagrams and on REE plots (Figs. 5.5, 5.8b). Colinear Harker diagram patterns characterize samplesfrom all units, disregarding the “enriched basaltic andesite of Nootka Sound”. All units display negative124FeO* SYMBOLS• Bonanza Group - Nootka Area• Bonanza Group - Alberni Area+ Bowen Island GroupA Harrison Lake Fm.* Middle Jurassic intrusionsTholeiitic44%•IAICaic-AlkalineMgONa+K2O8 I I..{!.. IIIAARhyolite 4 Corn/PanA‘‘ 70-0. 65 Rhyodacite/daciteTrachyteA60 - Andesite • TrAPhonolite55_ .r;j50-Bas/Trach/Neph45 - Sub-AB0 I I 111111 I I 111111 I 1111111 I 11111.001 .01 .1 1 10Zr/Ti02Figure 5.4- a) AFM diagram for samples of the Bonanza Group, Bowen Island Group, and Harrison LakeFormation. Note sample symbols in legend. b) ZriTiO2vs. Si02 compositional discriminate diagram.From Winchester and Floyd (1977).12515Il0+5 0C A•t9I,A AAA4SiO (wt %) 80 45 Si02(wt%)SYMBOLSO Bonanza Group - Ti enriched• Bonanza Group - Nootka Area• Bonanza Group - Alberni Area+ Bowen Island GroupA Harrison Lake Fm.* Middle Jurassic intrusionS..420 . A100145 SiO (wt %) 8015CC‘° : ‘H* •••AU 5 A *C•A01Aj45 Si02 (wt %) 8040OrCCCAA•AA41• 80045 Si02 (wt %)500 r-L ‘C100. C200•AA045Si02(wt%)15A AAIAH10p. ciAcUl AUAIA ACci CA800CoCDC ci•% • A•++ A2. (wt %)Figure 5.5 - Harker variation diagrams for samples from the Bonanza Group, Bowen Island Group, andHarrison Lake Formation. Note smooth linear to curvilinear trends for most elements, as well asdistinct separation of a subset of Nootka Sound samples (open squares).126correlations between A1203,FeO*, MgO, Ti02 and V and increasing silica content., The trace elements Zr,Nb, and Ta show weak positive correlations with increasing silica content (Fig. 5.5).Elemental scatter is evident in plots of K, Na, Ba, Ca, and other LIL elements against Si02,indicating partial remobilization of mobile constituents (Gill, 1981) Systematic compositional changes may beevaluated through the use of immobile compatible elements (Al, Ti, V, Cr) as monitors of progressive changewithin each of the rock suites (MacLean and Barrett, 1993; Fig. 5.6). A plot ofA1203versus Ti02demonstrates a linear relationship between rocks of the Bonanza Group (excluding the enriched basalticandesite), Bowen Island Group, and Harrison Lake Formation. A similar linear relationship exists betweenthe units on a 5i02 versus Ti02 plot, suggesting that the units have not undergone silica mobilization, whichwould lead to elemental scatter. The unique character of the enriched basalt of Nootka Sound is wellillustrated on the plots utilizing the immobile compatible element Ti02 as a monitor (Fig. 5.6a, b). Note theprogressive depletion ofTi02 at essentially constant levels of Si02 and A1203for concentrations of Ti02greater than 1.2 weight percent. This trend is interpreted to represent progressive ciystallization of Ti richphases, probably Fe-Ti oxides, prior to the ciystallization of more silica-rich phases. There is a distinctinflection point evident on the immobile compatible element diagrams between the horizontal linear trendsand the sloping trends; this inflection may result from fractional crystallization within a single magmachamber (i.e. a single fractionation sequence), or may indicate multiple magma chambers with distinct sources(i.e. two separate fractionation trends; Gill, 1981).Mobile-immobile incompatible element ratios monitor variations in magmatic sources (Pearce, 1982).Pairs of incompatible elements from a lava series related by fractional crystallization form a linear trend on abiaxial diagram that passes through the origin (Wood et al., 1979; MacLean and Barrett, 1993). Ta is chosenas an index of fractionation because it is an inunobile incompatible element that does not undergo enrichmentduring the subduction process, thus, assuming an originally homogeneous source, variations in theconcentration of Ta are simply the result of fractional crystallization (Wood et al., 1979; Pearce, 1982). Theincompatible elements Ce and Th display uniformity in within plate basalt and MORB environments12725I20 R AA+ DA15 oD0A+A10.5 1 1.5 2 2.5TiO(wt %)1009080 +70DQ0.5 1 1. 2 2.5TiO (wt %) SYMBOLS2 0 Bonanza Group- Ti enriched• Bonanza Group- Nootka Area• Bonanza Group- Alberni Area+ Bowen Esiand GroupA Harrison Lake Fm.E Middle Jurassic intrusionsFigure 5.6- a)Ti02 vs. A1203 variation diagram. Note horizontal fractionation pattern at Ti02 contents ofover 1.25 wt %., which corresponds to enriched basaltic andesite of Nootka Sound b) Ti02 vs. Si02variation diagram. Note similar pattern of Ti02 fractionation.. Note distinct inflection points betweenhorizontal and sloped fractionation trends.128(i.e. depleted mantle source), but display demonstrable enrichment in arc settings, suggesting enrichmentduring the subduction process (Wood, 1979; Pearce, 1982). Therefore plots of the incompatible elements Ce orTh against Ta may be to used to discriminate enrichment processes in island arc settings (Pearce, 1982). Theincompatible elements Ta, Ce, and Th are normalized to Yb because the behavior of an evolving system ismore easily evaluated by examining the changes in relationships between elements rather than changes in theabsolute concentrations of the elements. In addition, use of Yb adds an additional monitor of mantleheterogeneity, as it is not enriched by subduction processes, and variations in its value are related solely toprogressive variations in original source composition (Pearce, 1982).Ce/Yb and Tb/Yb are plotted against Ta/Yb on figure 5.7. Note the strong colinear trends evident inboth plots, and the transition from Bonanza Group samples close to MORB values toward Harrison LakeFormation samples at the more evolved end of the spectrum. Th, Ce, Ta, and Yb concentrations progressivelyincrease from the Bonanza Group to the Harrison Lake Formation, and reflect either incompatible behaviorduring fractional crystallization, or the gradual influx of incompatible elements to the system from a cmstalsource. Deviations away from the mantle array suggest minor enrichment of Ce and more substantialenrichment of Th relative to MORB/depleted mantle values. Differences in the degree of enrichment areprobably the result of the higher mobility of Th relative to Ce. The enrichment in Ce is uniform for thecompositional range, whereas there is a slight increase in the degree of Th enrichment from the mafic to thefelsic end of the range. This shift in Th enrichment may be the result of additional hydrous flux from thedescending slab or a minor degree of crustal contamination. Incompatible element ratio diagrams permitestimation of both the initial mantle composition and the degree of enrichment via hydrous flux duringsubduction (Pearce, 1982). The strong colinear trends between the Bonanza Group, Bowen Island Group, andHarrison Lake Formation are consistent with the evolution of a single magmatic system. If these suites are notderived from a single magmatic system, then each of the individual source regions must have undergoneidentical degrees of enrichment from the subducting slab, interacted with lower crust to the same degree, andbe initially derived from a homogeneous mantle.129I I I I7I.£1.I.1OEc_). I PrimordialMantle.0 2 I (a)idbII I I I10Ta/Yb10 I 111111 I I III IA1”1.,///1• /(b)III 11111 I II.01.01.1 1 10Ta/Yb SYMBOLSD Bonanza Group - Ti enriched• Bonanza Group - Nootka Area• Bonanza Group - Albemi Area+ Bowen Island GroupA Harrison Lake Fm.Middle Jurassic intrusionsFigure 5.7- a) Ta/Yb vs. Ce/Yb ; b) Ta/Yb vs. Tb/Yb, after Gill (1981). Note strong colinear trends andoverlap of all units.130B. Rare Earth ElementsRare earth element patterns from the Bonanza Group, Bowen Island Group, and Harrison LakeFormation vary from flat (LaN/YbN=1.O) to light rare earth element (LREE) enriched (LaN/YbN=6.O; Fig.5.8). REE patterns from each unit are consistent with range of compositions from low K tholeiite to mediumK calcalkaline (Gill, 1981). Each unit displays a mixture of REE patterns that may be subdivided into distinctgroups (Fig. 5.8). Figure 5.8 is a series of REE diagrams from each of the sample areas under consideration;the upper diagram displays the entire suite, and the lower diagram displays representative samplescharacteristic of each REE group. The REE patterns may be subdivided into three distinct groups:I. Flat BEE patterns (LaN/YbN=1.2-1.9). This group is characterized by flat to veiy slightly LREEenriched REE patterns with abundances lO-12x chondrite, and may contain minor positive Eu anomalies(Figs. 5.8, 5.9a). Group I (Fig. 5.9) represents a small percentage of all samples from the Bonanza (n=4) andBowen Island (n=1) groups, and is present in the Harrison Lake Formation (n=2) with slightly higher totalREE abundances than the rocks to the west. The flat REE patterns and low abundances suggest that a portionof each unit is primitive and has not interacted with the any crustal material or enriched component derivedfrom the subducting slab.II. LREE enriched BEE patterns (LaN/YbN=3.O-7.O). This group is characterized by LREEenriched patterns with LREE abundances 20-70x chondrite (Fig. 5.8, 5.9b). REE pattern generally display anegative slope, and minor HREE enrichment is evident in part. There is an overall increase in LREEenrichment from the Bonanza Group east to the Harrison Lake Formation, but there is no direct correlationbetween Si02 content and LREE ennchnient. A second subset occurs within LREE enriched Group II,consisting of LREE enriched patterns (LaN/YbN=3.O-4.5) with a distinct concave upward appearance. Thissubset is restricted to volcanic rocks from the Harrison Lake Formation and associated plutonic rocks to thewest.13100—(-1 0C’=©Cl)22© zC’ 0..I’,— VZ V C9__oo 0 o oo— 0—upuoJqJ/dwIsal 000•0S 00— zZ C’ —©-I)0C 0 0 000——upUO1qJ/dLUU aflJpuoJq3/dwusFigure 5.8 - Rare earth element diagrams for all samples, piotted according to sample area. Top diagram ineach set is total sample suite, lower diagram is representative sample of each subgrouping of REEpatterns, a) Port Alberni area; b) Nootka Sound area. Enriched basaltic andesite shown with opensquares.132LETsainbsundofl1Muots!spusqpipuuuonuuojirjuosu.m(p‘dnoipujsjuoa(csu.rnidJOuidnoi&jnstpjoiduisATW1us1d1S!ureiupiaoi‘irns1dun?smoS!stpU!Ufl!idoiarujdunsoupioxpoid‘sjdunsjjiojsun?x!puuip‘p.-sc.“LtISample/ChrondriteSample/Chrondrite-C_—CCCCOa-ZZ a.a.0cr1C)C)— a.a.z1111111)Sample/ChrondriteSample/Chrondrite——-—CC— —CCC‘-azVa.B00B‘1C) a.0_______Group I- Flat REE4è 100(a)1La Ce Pr Nd Sm Eu Gd Th Dy Ho Er Tm Yb LuGroup II- LREE enriched100I i4I4EI11.1(b)La Ce Pr Nd Sm Eu Gd Tb Dy Ho & Tm Yb LuGroup III- High Flat REE10011Occ)La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb LuFigure 5.9 - Comparison of REE subgroups from each sample suite. Note strong similarities between patternsfrom all units in Group I and Group II. Group III corresponds to enriched basaltic andesite fromNootka Sound, and does not occur in other sample suites. Symbols as in figure 5.8.134III. Flat REE patterns with high total REE (LaN/YbN=1.O-2.O). This group has a flat to slightlyLREE enriched REE pattern with relatively high REE concentrations of 20-50x chondrite (Fig. 5.8, 5.9c).This REE pattern is unique to samples from Nootka Sound, and corresponds entirely to the enriched basalticandesite suite. The flat BEE patterns from the Harrison Lake Formation have elevated PEE patterns similar tothe enriched basaltic andesite of Nootka Sound, but the Harrison Lake samples are placed in Group I instead ofGroup III due to their low Ti02,FeO*, and Zr values compared to samples from Nootka Sound.The Bonanza Group, Bowen Island Group and Harrison Lake Formation contain similar groups ofBEE patterns. Group I and Group II have representatives in each unit, while Group III is constrained to theNootka Sound area (Fig. 5.9a, b, c). The consistency of BEE patterns among the units suggests that each unitcontains rocks derived from similar batches of magma. Group I rocks tend to have high MgO values (>6 wt%), and may represent a primitive “parental” magma common to each unit (Wilson, 1988). Group II rocksfrom each unit display LREE enrichment, with the magnitude of enrichment increasing from the BonanzaGroup to the Harrison Lake Formation. This gradual increase in LREE enrichment may reflect an increase incrustal contamination from west to east, or may be the result of progressive differentiation of a magmaticsystem.Group III BEE patterns are unique to the Nootka Sound area. These samples have anomolously highTi02,FeO*, Zr, Y, and HREE concentrations compared to other Bonanza Group samples, and must have adifferent source. Generation of basaltic andesite of this composition require elevating Ti02,FeO*, HFSelements (Zr, Y) and BEE element concentrations without fractionating the BEE or raising Si02 or K20contents. Significant assimilation of countly rock would elevate Si02 and K20 content and fractionate theREE. Direct derivation from a depleted mantle source (similar to MORB) does not account for the IIFS andBEE enrichment relative to MORB, and does not explain the spiked incompatible trace element pattern.Addition of a subduction component to a depleted mantle wedge would not elevate the FIPSE or BEEsufficiently (McCulloch and Gamble, 1991).135Barrie et al. (1991) describe enriched basalts associated with the Kamiskotia Gabbroic Complex,western Abitibi Subprovince, Ontario, Canada, that are remarkably similar to the enriched basaltic andesitesuite from Nootka Sound. Barrie et al. (1991) effectively modeled the generation of the enriched basaltthrough 70-80% fractional crystallization of a partial melt derived from depleted mantle (MORE-like) source.The most plausible explanation for the enriched basaltic andesite of Nootka Sound is derivation from adepleted mantle source coupled with some degree of partial melting of Karmutsen Formation basalt.Karmutsen Formation basalts have Si02 contents < 50 weight percent, relatively high FeO* and Ti02, andflat to slightly enriched PEE patterns up to 13x chondrite. Interaction of magma derived from a depletedmantle wedge with gabbroic to basaltic crust near the base of the Karmutsen Formation would sufficientlyelevate the Fe04,Ti02,HFSE and PEE evident in the Nootka Sound samples. Assimilation of small volumesof lower Kartmutsen crust, perhaps coupled with periodic recharge from the depleted mantle source, wouldelevate HFSE and PEE without raising Si02 content (Barrie et al., 1991).5.7 ISOTOPIC SIGNATURESNd and Sr isotopic analyses were conducted on representative suites from the Bonanza Group, BowenIsland Group, and Harrison Lake Formation, including 18 samples from the Nootka Sound area, 3 samplesfrom Bowen Island and exposures to the north, and 14 samples from the Harrison Lake area (Table 5.2).Analytical techniques are discussed in Appendix A.Nd and Sr values for the Bonanza Group, Bowen Island Group and Harrison Lake Formationdemonstrate the juvenile nature of each of the units. Initial ENd values generally range from +3.5 to +7.5, andinitial 87Sr/6rvalues cluster between 0.7030 and 0.7045. One sample from the Bonanza Group displays ananomolously high ENd value ( Nd=11.38). Two of the Harrison Lake Formation samples have anomolouslylow strontium concentrations, which lead to initial strontium values that fall below the mantle array, andsuggest disturbance of the Rb/Sr isotopic system via Sr loss. Isotopic values partially overlap between units,13610. I1IFigure 5.10- ENd vs 87Sr/6rdiagram for all samples. Note tight cluster of data points, with distinct shiftto lower more evolved values in Harrison Lake Formation. MORB = mid-ocean ridge basalt; IAVu =uncontaminated island arc volcanic rocks; IAVc = contaminated island arc volcanic rocks; CHUR =Chondritic Uniform Reservoir (DePaolo and Wasserburg, 1976).I”AENdA Bonanza Group• Harrison Lake Fm+ Bowen Island Group Im’IAVuIAVcCHUR1III-10.000.7020 0.7040 0.7060 0.708087Sr/8óSr0.7100137Age87Sr86Sr(i)+1-RbSr7RbI86S87Srf86Sr(i)SmNdSm/Ndf143/144+1-EpsilonNdEpNd(i)HarrisonLakeFormationVolcanios‘-‘iO4jbsn921750.704426137.25182.230.120.7041393.6512.090.1849-0.0600.51292365.565.83116jbm921750.7050111825.68268.030.280.7043213.8116.220.1414-0.2810.51283443.824.96142jbm921750.7136812666.7739.754.860.7015883.7616.000.1419-0.2780.51282443.634.85gl5ljbsn921750.7051641314.45168.710.250.7045476.2523.820.1590-0.1920.51290495.196.03287jbm921750.705057II35.93317.530.330.7042413.4615.030.1399-0.2890.51283893.905.17291jbm921750.7066831048.62122.091.150.7038165.6518.950.1801-0.0840.512800113.163.53307jbni921750.7041902617.3878.310.640.7025924.6016.980.1626-0.1730.512921115.526.2813jbm931750.704005171.16218.060.020.7039672.9813.030.1387-0.2370.51291555.406.7014jbm931750.704422162.68201.190.040.7043262.1810.350.1274-0.3520.512916105.426.9715jbm931750.7068204957.97151.421.110.7040645.5020.440.1627-0.1730.512858134.295.0517jbm931750.704325210.92115.080.020.7042684.1213.950.1786-0.2370.51293265.746.7821jbm931750.7044641725.32323.940.230.7039014.4217.220.1552-0.2110.51283993.924.85.22jbm931750.7054641922.96136.920.490.7042563.1814.420.1332-0.3230.51282234.766.1825jbm931750.7056281338.98153.910.730.7038053.1314.940.1267-0.3560.51288984.906.46BowenIsland.GroupVolcanics0090jbm931850.7035282216.62359.900.130.7031762.8611.310.1533-0.2210.512891104.945.9691jbm931850.7038083211.81628.820.050.7038082.4.609.150.1631-0.1710.5129656.287.08;. OBonanzaGroupVolcanios92-1451950.704100167.50205.340.110.7038072.2907.480.1847-0.0610.512957136.226.5392-1531950.703401183.32127.900.080.7031921.0602.120.30160.5330.513198810.928.3292-1691950.7040511523.02319.970.210.7034742.6908.740.1862-0.0530.51297546.576.8492-1701950.7047751935.69264.420.390.7036927.8926.570.1795-0.0870.513011107.287.7192-177l950.7049571564.59451.600.410.7038102.8010.590.1590-0.1920.5131732610.4411.3892-1851950.7084361259.6194.381.830.7033688.1426.900.1808-0.0810.51293995.876.2792-1871950.7067632059.63153.801.120.7036525.8419.980.1768-0.1010.51300197.087.5892-2351950.7065841961.71178.851.000.7038158.6331.620.1650-0.1610.51296756.427.2192-2411950.7064741353.41195.970.790.7042875.5320.090.1655-0.1590.51293975.876.65l92-2441950.7042351825.52280.890.260.7035065.1417.160.1802-0.0840.51297286.526.9392-2491950.7042978732.60308.610.310.7034502.2007.300.1821-0.0740.51297246.526.8892-2521950.7038711522.98282.440.240.7032185.4419.390.1697-0.1370.51299576.967.6492-2561950.704237176.71128.000.150.7038175.4519.360.1796-0.0870.51295756.226.6592-2581950.7044601929.75309.800.280.7036894.5715.730.1753-0.1090.51296796.426.95.92-25919559.07324.250.534.2517.920.1432-0.2720.51286994.515.8492-2611950.7079182049.78104.681.380.7041025.0218.410.1652-0.1600.51292655.626.4092-2701950.7044771247.50396.830.350.7035164.3317.480.1499-0.2380.51283763.885.0592-2721950.7042731534.65455.550.220.7036622.8310.870.1576-0.1990.51285854.295.27and are within the range of uncontaminated island arc volcanic rocks (Fig. 5.10). The isotopic values plotvery near the MORB field, demonstrating a strong depleted mantle component in the source region(McCulloch and Gamble, 1991). Initial Sr values are displaced to the right of the mantle array, consistentwith an influx of evolved Sr from seawater-altered oceanic crust in the descending slab (White and Patchett,1984; McCulloch and Gamble, 1991).A comparison of ENd withfSm/Nd’ an expression of the Sm/Nd ratio of the rock relative tochondritic values (1M =(‘47Sm/ Nd)sple/’47Sm/’4CHrjR- 1; Shirey and Hanson, 1986),evaluates the LREE enrichment of a rock suite relative to its isotopic signature (Fig. 5.11). Differences inLREE enrichment between rock suites correspond to differences in fSnvNd values, resulting in verticaldisplacement of points on this diagram. LREE enrichment results in lower values, and LREE depletion resultsin higher values. The restricted isotopic range of the Bonanza Group, Bowen Island Group, and HarrisonLake Formation contrasts markedly with the range of LREE enrichment evident in them. Note that, whilethere is significant overlap between the rock units, the Harrison Lake Formation is generally more LREEenriched than the Bonanza Group over the same range of isotopic values. The higher degree of LREEenrichment in the Harrison Lake Formation compared to the other units could be the result of fractionalcrystallization of a single magmatic system, differences in the type of crust the magmas interacted with, orentirely separate magmatic histories. The gradational nature of the LREE enrichment coupled with therestricted range of isotopic values argues for similar source characteristics of the Bonanza Group, BowenIsland Group, and Harrison Lake Formation.Bonanza Group, Bowen Island Group, and Harrison Lake Formation isotopic values partially overlapin ENd-87Sr/6risotopic space, but there is a distinct shift to lower ENd and higher87Sr/6rvalues fromthe Bonanza Group to the Harrison Lake Formation. The majority of the Bonanza Group and Bowen IslandGroup rocks have initial ENd values within the range of MORB, and that the Harrison Lake Formation valuesare distinctly shifted to lower Nd values by up to 3 8Nd units. Island arc volcanic rocks commonly display adownward shift in 8Nd values relative to MORB/depleted mantle values; this shift is normally attributed to1390.20___________________________________________________A Bonanza Group• Harrison Lake Fm+ Bowen Island Group MO1U3:0.00 -I LREEI4ennchment ARocks020 1.•0ENdFigure 5.11-versus ENd diagram for all samples. Note wide range of LREE enrichment as opposedto tight range of 8Nd values.140sediment subduction and consequent enrichment of depleted mantle in the mantle wedge (White and Patchett,1984; McCulloch and Gamble, 1991). An increase in sediment subduction through time would explain theshift in SNd values between the Bonanza Group and the generally younger Harrison Lake Formation, or thedifference may be the result of magmnatic interaction with different types of crust. The latter hypothesis issupported by the presence of an inherited component evident in the U-Pb systematics of zircon derived fromrhyolite in the upper portion of the Harrison Lake Formation (Chapter 4). Distinctly concave upward PEEpatterns evident in a subset of Group II PEE patterns also support some level of crustal interaction in theHarrison Lake Formation (Hanson, 1980).The proposed partial melting of lower crust to explain the isotopic shift between the Bonanza Groupand Harrison Lake Formation does not preclude crustal involvement in the generation of Bonanza Group andBowen Island Group rocks. There is ample evidence of partial melting of lower crust in the Bonanza Group,including abundant amphibolitic inclusions in the comagmatic Westcoast Complex (Debari and Mortensen,1994), Paleozoic inheritance in zircon from the comagmatic Island Intrusions (Parrish and McNichol, 1993),and the presence of the enriched basaltic andesite of Nootka Sound. The primary difference in the isotopicsignature between the different units is probably not the degree of partial melting, but a difference in the typeofcrust that the ascending magma interacted with. The crust beneath the Bonanza Group and Bowen IslandGroup consists of Paleozoic juvenile arc material and Early Mesozoic Kartmutsen Formation basalt, whereascrust beneath the Harrison Lake Formation is Triassic ocean floor and an unknown quantity of older material(Friedman and Cui, 1994).5.8 CONSTRAINTS ON ARC CORRELATIONA. Lithostratigraphic ConsiderationsThe Bonanza Group and Harrison Lake Formation both display rapid lateral and vertical facieschanges, and consist of interdigitated lava flows, breccias, tuffbreccias, lapilli tuff tuff and associated141volcaniclastic sedimentary rocks. The four-fold subdivision applied to the Harrison Lake Formation (Arthur,1987, 1993; Mahoney et al., 1994; Chapter 4) is comparable with local subdivisions evident in the BonanzaGroup (Jeletsky, 1976; Nixon et al., 1994); these subdivisions are readily applicable over 10’s of squarekilometres, but break down over larger areas. Correlations of these successions must therefore be based oncontact relations with adjacent strata and permissible age constraints.The gradational contact at the base of the Bonanza Group represents the initiation of volcanism inEarly Jurassic time. Nixon et al.(1994) propose a two-fold stratigraphy on northern Vancouver Island,consisting of a lower, regionally persistent, predominantly subaqueous epiclastic-pyroclastic succession thatshoals upward into a subaerial, laterally variable, succession composed of lava flows and associated pyroclasticrocks. The subaerial rocks comprise the vast majority of the section (Nixon et al., 1994). The coarseningupward succession is interpreted by Nixon et al. (1994) to represent the gradual emergence of a volcanic islandarc from submarine to subaerial in the Early Jurassic. The predominance of Sinemurian and Pliensbachianmarine fauna intercalated with volcanic strata in the Bonanza Group suggests at least part of the arc wasintermittently inundated by marine waters until at least the Late Pliensbachian.The Bowen Island Group contains interbedded black argillite and varicolored tuff intercalated withvolcanic flows. The sediment-dominated northwestern portion of the outcrop belt contains probableSinemurian fossils, and the volcanic dominated southeastern portion contains Toarcian rhyolite flows.Structural complexities and intrusion preclude accurate stratigraphic assessment, but the facies distribution inthe Bowen Island sequence is consistent with tuffaceous marine strata overlain by proximal volcanic strata, ina manner similar to the Bonanza Group.The basal conglomerate of the Harrison Lake Formation records localized uplift and erosion in EarlyJurassic time, followed by sub-wave base deposition of marine fine-grained clastic rocks. Tuff intercalated withthe marine strata indicate initiation of volcanism in the late Early Toarcian. Thin bedded, fine grained marinestrata grade upward into thick bedded coarse grained tuffaceous sandstone and conglomerate, overlain by142volcanic breccia and lava flows. This coarsening upward succession in the Harrison Lake Formation recordsthe gradual emergence of a volcanic island arc in Early to Middle Jurassic time.It is suggested herein that the initial pyroclastic and epiclastic sedimentation in the Harrison LakeFormation may correspond with active subaerial volcanism in the Bonanza Group. Nixon et al. (1994)document ash flow tuff sheets and caldera collapse features indicating explosive volcanism in the BonanzaGroup. Tuffaceous strata in the lower (?) Bowen Island Group and lower Harrison Lake Fonnation may bedistal equivalents to ash flow deposits in the Bonanza Group. Similarly, active submarine and subaerialvolcanism in the Harrison Lake Formation may correspond to the pyroclastic and epiclastic successionoverlying volcanic rocks of the Bonanza Group (Jeletsky, 1976; Muller et al., 1981).Strata of the Bonanza Group and Harrison Lake Formation are unconformably overlain by shallowmarine clastic strata of Callovian age.B. Temporal ConsiderationsAge relations between the Bonanza Group, Bowen Island Group, and Harrison Lake Formation canbe used to infer the time transgressive development of a volcanic arc system. Initial development of the arcbegan in Sinemurian time in the Bonanza Group, and emergence of the Bonanza Group as a subaerial edificemay correspond to the delivety of air fall tuif to Sinemurian strata of the Haibledown Formation and lower C?)Bowen Island Group (ripper et al., 1991). Continued development of the volcanic edifice may have beenresponsible for delivery of conglomeratic debris to the base of the Harrison Lake Formation (Celia CoveMember), and perhaps was a causative factor in the subsidence evident in the lower Harrison Lake Formation(Francis Lake Member). Late early Toarcian pyroclastic activity in the Bonanza Group and/or Bowen IslandGroup may have supplied tuffaceous material to the lower Harrison Lake Formation.143Eastward migration of the main locus of volcanism may have progressed from Sinemurian to LatePliensbachian (or younger) in the Bonanza Group to Toarcian in the Bowen Island Group, to Aalenian to LateBajocian in the Harrison Lake Formation. This eastward migration of volcanism encompasses 30-35 Ma,which is a time frame comparable to long-lived Tertiary systems of the western Pacific (Park et al., 1990)C. Geochemical ConsiderationsThe overall geochemical coherence of the Bonanza Group, Bowen Island Group, and Harrison LakeFormation indicates the units share vezy similar magmatic histories. Systematic trends on Harker diagramsand immobile compatible element plots may be attributed to progressive differentiation of the magmaticsystem (Pearce, 1982; Wilson, 1988; MacLean and Barrett, 1994). These systematic trends are not definitiveindicators of a cogenetic relationship, but are consistent with derivation of the Bonanza Group, Bowen IslandGroup, and Harrison Lake Formation volcanic rocks from similar sources, perhaps within a single magmaticsystem. There is an overall trend from tholeiitic to calcalkaline andesitic basalt and andesite in the BonanzaGroup to calcalkaline andesite to rhyolite in the Harrison Lake Formation. Incompatible trace elementpatterns for all units display enrichment of LIL elements relative to HFS elements, and the LILEIFIFSE ratiosystematically increases from the Bonanza Group through to the Harrison Lake Formation. Incompatibleelement ratio diagrams suggest similar degrees of source enrichment for each unit, and are consistent withfractionation of a magmatic system derived from a depleted mantle source (Pearce, 1982). Similar groups ofPEE patterns are displayed by each unit, suggesting derivation from different batches of magma displayingsimilar source characteristics.The primary difference between the units is the existence of enriched basaltic andesite in the NootkaSound region, which is attributed to partial melting of basaltic lower crust, probably the Karmutsen Formation.The Karmutsen Formation basalt has not been documented beneath the Bowen Island Group or the HarrisonLake Formation, units which do not contain enriched basaltic andesite.144D. Isotopic ConsiderationsNd and Sr isotopic data for the Bonanza Group, Bowen Island Group and Harrison Lake Formationdemonstrate the juvenile nature of the volcanic rocks, and strongly suggest magma derivation from anuncontaminated depleted mantle source (McCulloch and Gamble, 1991). Displacement of initial Sr values tothe right of the mantle array indicates similar levels of Sr enrichment in each of the units (White and Patchett,1984).Evaluation of fSmiNd values against ENd demonstrates progressive LREE enrichment from theBonanza Group to the Harrison Lake Formation (Shirey and Hanson, 1986). This LREE enrichment may bethe result of fractional crystallization, or the result of magma interaction with different crustal sections.Fractional crystallization, however, does not account for the initial difference in 6Nd values between theBonanza Group and the Harrison Lake Formation. The observed isotopic shift is greater than the variationexpected from a homogeneous source, and suggests differing degrees of crustal interaction between theBonanza Group and Harrison Lake Formation. The isotopic and geochemical data are consistent withformation of the Bonanza Group, Bowen Island Group, and Harrison Lake Formation in a single volcanic arcsystem. Isotopic and geochemical variations across the arc result from the ascent of magma derived from anuncontaminated depleted mantle wedge through different crustal sections.E. Structural ConsiderationsStructural evidence for a common history for the Bonanza Group, Bowen Island Group, and HarrisonLake Formation includes evidence for contemporaneous deformation, intrusion by coeval plutons along themargins of the Coast Plutonic Complex, and geophysical constraints.Each of the units displays evidence of pre-Callovian structural deformation. The Bonanza Group andunderlying Vancouver Group are locally overlain with angular unconformity by Callovian strata of the145Kyoquot Group in west-central Vancouver Island (Muller et al., 1974). Elsewhere on the Island, the BonanzaGroup is unconformably overlain by Lower Cretaceous strata. East of Vancouver Island, a ductile fault cuttingpost-Karmutsen strata on Quadra Island is intruded by a 163.8 Ma pluton (Monger and McNichol, 1994),suggesting post-Late Triassic, pre-Callovian ductile deformation. Isoclinally folded Bowen Island Groupstrata are cut by 155-160 Ma plutons, arguing for post-185, pre-155 Ma deformation (Monger, 1993:Friedman and Armstrong, 1994). Late Bajocian to early Bathonian strata of the Harrison Lake Formationcontain mesoscopic tight folds and overturned bedding that are absent in overlying Callovian strata.Wrangellia and Harrison terranes are separated by Jurassic and Cretaceous plutons of the CoastPlutonic Complex. The eastern edge of Wrangellia is intruded by Middle to Late Jurassic plutons as old as178 Ma (Webster and Ray, 1990), coeval with the Island Intrusions, which are considered comagmatic to theBonanza Group. The western edge of the Harrison terrane is intruded by Middle Jurassic plutons and satellitestocks as old as 167 Ma (Friedman and Armstrong, 1994). The age of the plutons intruding the western edgeof the Harrison terrane overlap the age of a rhyolite dome in the upper portion of the Harrison LakeFonnation. The Bonanza Group and Harrison Lake Formation are both intruded by plutomc rocks that aregeochemicafly and isotopically identical to the coeval volcanics. The plutonic rocks are interpreted herein asthe plutonic roots to the Middle Jurassic volcanic system. The Middle Jurassic plutons are apparentlyrestricted to the eastern and western edge of the pre-90 Ma Coast Plutonic Complex (Friedman andArmstrong, 1994), with younger plutons forming margin-parallel belts in the interior of the complex. Ipropose that the current distribution of the Wrangellia and Harrison terranes is at least in part an artifact ofextension induced by the emplacement of younger plutons.Seismic reflection and refraction data in the southern Coast Belt suggest that Wrangellia andHarrison terranes and intervening plutons form a coherent crustal block (Zelt et al, 1991). Geophysical datademonstrate a distinct change in crustal velocity from rocks of Wrangellia and Harrison terranes to rocks tothe east in the vicinity of the Harrison Lake fault, but do not resolve any major crustal structures between theHarrison and Wrangellia terranes (Zelt et al., 1993; O’Leary et al., 1993). The suggestion that Wrangellia and146Harrison terranes form a coherent crustal block agrees with the observations of Journeay and Friedman (1993),who argue these rocks acted as a coherent structural block during Cretaceous compression.CONCLUSIONSThis investigation represents the first integrated study into the geology, geochemistry, and isotopiccharacteristics of the Bonanza Group, Bowen Island Group, and Harrison Lake Formation. Stratigraphic,geochemical, isotopic and age considerations suggest that Lower to Middle Jurassic strata of Wrangellia andHarrison terranes originally formed a contiguous arc sequence built upon different Triassic basements. Thearc was initiated in the Early Jurassic on Wrangellia, and the locus of volcanism swept eastward fromSinemurian to Bajocian time. Volcanism ceased in the Late Bajocian or earliest Bathonian, and was followedby post-early Bathonian, pre-Callovian deformational events of varying intensity. The formation of theBonanza-Harrison arc requires juxtaposition of Triassic rocks of Wrangellia and Harrison terranes prior to theEarly Jurassic. The Bonanza-Harrison arc represents the first stage of magmatism associated with the CoastPlutonic Complex magmatic arc.147CHAPTER 6REGIONAL TECTONOSTRATIGRAPHIC CORRELATIONS iN THE SOUTHERNCANADIAN CORDILLERA:iMPLICATIONS FOR JURASSIC TERRANE LINKAGES AND BASINEVOLUTION1486. REGIONAL TECTONOSTRATIGRAPHIC CORRELATIONS IN THE SOUTHERN CANADIANCORDILLERA: IMPLICATIONS FOR JURASSIC TERRANE LINKAGES AND BASINEVOLUTION6.1 INTRODUCTIONThe southwestern Canadian Cordillera is a complex mosaic of tectonostratigraphic terranesjuxtaposed by regional fault systems and intruded by Jurassic to Tertiary plutons (Fig. 6.1). The terranes havebeen described as island arc systems, oceanic floor assemblages, accretionary complexes, and other fragmentsof tectonic flotsam that were accreted to the western margin of North America in the Mesozoic (Coney et aL,1980). The Mesozoic paleogeographic distribution of the terranes is uncertain, as is the timing, sequence, andmechanism of their amalgamation to the continental margin. The pre-accretionaiy configuration of theterranes that presently comprise the southern Canadian Cordillera has important implications for the tectonicevolution of the region.A number of models of tectonic evolution have been proposed to explain the amalgamation ofterranes in the Canadian Cordillera. These models fall into three general categories:1) early Late Cretaceous accretion of the Insular Superterrane (Terrane II of Monger et al. (1982),comprising amalgamated Wrangellia, Alexander, and Peninsula terranes) with previously accreted terranes ofthe Intermontane Superterrane (Terrane I of Monger et al. (1982), primarily Cache Creek, Quesnellia, andStikinia terranes; Monger et a!., 1982; Garver, 1992; Garver and Brandon, 1994);2) Middle to Late Jurassic dextral translation and oblique transpression of Insular Superterrane withconcomitant development of transtensional rift basins along the western margin of the IntermontaneSuperterrane (Gehrels and Saleeby, 1985; Saleeby and Busby-Spera, 1992; McClelland et al., 1992);149QuesnelliaCache CreekStilciniaKootenayCassiarCoast Plutonic Complexundivided metasedimentsof the eastern Coast BeltFigure 6.1- Generalized terrane map of the southern Canadian Cordillera, showing major structures andintrusive complexes. Inset map shows location within morphogeologic belts of the Canadian Cordillera.////////// AAAAAAAIAAAAAA/ // / / // / / // / // / // / 7/ / /// / / 7/ / /•/ / / // / /•/ / / 7/ / // / / // / /WrangelliaHarrisonCadwalladerBridge RiverMethowAAAI\terranes of Northwest CascadesS—I 3Omineca Crystalline ComplexAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA A AAAAAAAAAAAAAAAA,AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AA AA A A AAAAAAAAAAAAAA\AAAAA,AA...uIILc.WASH1503) Middle Jurassic accretion of all terranes and the development of an Andean-Sierran typecontinental arc, accompanied by Late Jurassic to early Late Cretaceous intra-arc rifting and basin development(van der Heyden, 1992). Critical evaluation of any of these proposed models requires definitive evidence ofterrane linkages.Terranes of the southern Canadian Cordillera include, from west to east, Wrangellia, Harrison,Cadwallader, Bridge River, Methow, and Quesnellia. Each of these terranes contains Lower to MiddleJurassic volcanic arc rocks or volcaiuclastic sediments that overlie Triassic and older basement and areoverlain by Cretaceous coarse-grained clastic strata (Fig. 6.1). Most workers agree that Lower Cretaceouscoarse clastic rocks, particularly polymict, granitoid-bearing conglomerates, provide a stratigraphic linkamongst all the terranes. Earlier stratigraphic linkages are more tenuous, and are the subject of thisinvestigation. Links between the Harrison, Cadwallader, Bridge River, and Methow terranes are of particularimportance, because these terranes straddle the boundary between rocks traditionally assigned to the Insularand Intermontane Superterranes (Monger et al., 1982; Gehrels and Saleeby, 1985; van der Heyden, 1992).This investigation examines the lithostratigraphy, biostratigraphy, volcanic geochemistry, Nd and Srisotopic characteristics, and depositional setting of Middle Jurassic strata in the southern Canadian Cordillera.These data are used to evaluate the stratigraphic evolution of each terrane, document geologic similarities ordifferences between terranes, propose stratigraphic correlations, and develop a comprehensive basin evolutionmodel for Middle Jurassic strata in the region.6.2 GEOLOGIC SETTINGMiddle Jurassic strata in the southern Canadian Cordillera occur within Wrangellia, Harrison,Cadwallader, Bridge River, Methow, and Quesnellia terranes (Figs. 6.1, 6.2). Late Jurassic to Tertiarystructural deformation and magmatic activity have obscured the original structural and stratigraphicrelationships between terranes. The current distribution of terranes is controlled primarily by regional151structures of Late Cretaceous to Tertiary age and by Late Jurassic to Tertiary plutons (Monger and Journeay,1992, 1994; Journeay and Monger, 1994; Figs 6.1, 6.2; Plate 1).Plutomc rocks locally comprise over 80% of all exposures, particularly in the southwestern Coast Belt(Fig. 3.2, Plate 1). Early Jurassic and older plutons are found primarily in Quesnellia and Wrangellia. Middleto Late Jurassic plutons intrude Wrangellia, Harrison, and Quesnellia terranes, but have not been documentedin the Methow, Bridge River, and Cadwallader terranes. Late Early Cretaceous to Early Tertiary platonicrocks form broad northwest-trending belts that intrude all terranes and cut terrane boundaries throughout theregion (Friedman and Armstrong, 1994). Metamorphic grade varies throughout the region, but rarely exceedslower greenschist grade, with the exception of amphibolite grade rocks in the imbricate zone of the Coast BeltThrust System (Journeay and Friedman, 1993).The structural setting of the southern Canadian Cordillera is dominated by post- 100 Ma structures;older structures exist, but their timing is difficult to resolve. Pre-100 Ma, terrane specific structures are knownonly from Quesnellia and Wrangellia, although Late Triassic blueschist occurs in melange of the Bridge Riverterrane (Monger and Journeay, 1992; Archibald et aL, 1990). Late Early Cretaceous to Tertiary west- andeast-directed contractional faults, dextral strike-slip faults, and minor sinistral and extensional faults disruptinternal terrane stratigraphies and cut across terrane boundaries (Wheeler and McFeely, 1991; Journeay andMonger, 1994; Plate 1).A. Terrane DistributionWrangellian strata are separated from smaller terranes to the east by Middle Jurassic to Tertiarymagmatic suites of the Coast Plutonic Complex. Correlations between Middle Jurassic volcanic arcassemblages of Wrangellia and Harrison terranes were proposed in the preceding chapter. Between the CoastPlutonic Complex and the Yalakom-Hozameen and Fraser fault systems (Fig. 6.2), portions of the Hairison,Cadwallader, Bridge River and Methow terranes are imbricated within the late Early Cretaceous west-directed1520Figure 6.2 - Schematic geologic map of the eastern Coast Belt and western Intermontane Belt, showingdistribution of Lower to Middle Jurassic strata. Compare to Fig. 3.2.1530I0-.EI0I..C00/1tNC. , I CL—I (1CI—PacificRimInsularBeltCoastBeltIntermontaneCD0 ‘p 0’) C) cn CD C) 0 CD — CD IWrangelliaCadwalladerBridgeRiverILBeltakmOffshoreVancouverMainlandB.C.Islandkilometres100155Figure 6.4 - Time-stratigraphic sections of major terranes in the southern Canadian Cordillera.Coast Belt Thrust System (Journeay and Friedman, 1993). Rocks of the Harrison, Cadwallader and BridgeRiver terranes are interpreted to be protoliths for imbricate thrust nappes of high-grade metamorphic rocks inthe central portion of the Coast Belt Thrust System east of Hamson Lake (Figs. 6.2, 6.3). Cadwallader andBridge River terranes are complexly interdigitated along transcurrent faults in the eastern Coast belt, west ofthe Yalakom and Fraser faults (Journeay and Monger, 1994; Schiarizza et aL, 1990; Figs. 6.2, 6.3). TheMethow terrane occupies a hinterland position to the Coast Belt Thrust System, and is largely separated fromthe system by the Yalakom-Hozameen and Fraser fault systems. The Pasayten fault separates the Methowterrane from predominantly Triassic and Jurassic arc rocks and plutons of the Quesnellia terrane.6.3 STRATIGRAPHIC CHARACTERIZATIONA. Harrison Terranea. Terrane DescriptionThe Harrison terrane consists of greenstone, chert, limestone, argillite and greywacke of the TriassicCamp Cove Formation unconformably overlain by Lower to Middle Jurassic (Toarcian to Bathonian) volcanicand volcarnclastic strata of the Harrison Lake Formation (Figs. 6.1, 6.2). Fine-grained sandstone, siltstoneand shale of the Middle Jurassic (Callovian) Mysterious Creek Formation unconformably overlie the HarrisonLake Formation, and are gradationally overlain by fine-grained volcaniclastic strata of the Upper Jurassic(Oxfordian) Bilihook Creek Formation. Capping the sequence is polymict, granitoid-bearing conglomerate ofthe Lower Cretaceous (Berriasian to Valanginian) Peninsula Formation and overlying volcanic rocks of theBrokenback Hill Formation (Valanginian to Albian). The latter two formations comprise the Gambier Group,a distinctive stratigraphic sequence mapped throughout the southern Coast Mountains (Wheeler andMcFeeley, 1992; Journeay and Monger, 1994).156Lower and Middle Jurassic strata of the Harrison Lake Formation comprise the majority of theHarrison terrane, constituting the southern 60% of its area and over 50% of its composite stratigraphicthickness (Figs. 6.2, 6.4). The Harrison Lake Formation has been discussed in detail in Chapter 4, and willonly be briefly reviewed here.b. LithostratigraphyThe Harrison Lake Formation is subdivided into four members, including the Celia Cove, FrancisLake, Weaver Lake, and Echo Island Members (Arthur, 1987; Arthur et aL, 1993; this study; Fig. 6.5). Thebase of the formation is an angular unconformity with underlying greenstone, chert, graywacke and argillite ofthe Middle Triassic Camp Cove Formation. The Celia Cove Member consists of a fining upward sequence(50+ m) of polymict conglomerate, sandstone, siltstone, and shale. Subangular conglomerate clasts near thebase of the member are primarily derived from the subjacent Camp Cove Formation, although well-roundedPennian limestone clasts in the unit may be derived from the Chilliwack terrane to the south (Monger, 1985;Arthur, 1987; Arthur et al., 1993). Subrounded conglomerate clasts higher up in the sequence arepredominantly intermediate volcanic rocks. The Celia Cove Member is gradationally overlain by sandstoneand siltstone of the Francis Lake Member (Fig. 6.5).The lower 15-30 m of the Francis Lake Member fines upward from conglomerate and sandstone ofthe subjacent Celia Cove Member into a 15-20 m succession of ainmonite-bearing shale and siltstone. Abovethe argillaceous succession, the Francis Lake Member becomes coarser grained and thicker bedded, andcomprises tuffaceous lithic wackes, siliceous siltstone, ciystal vitric tuff and lapilli tuff. The Francis LakeMember is conformably overlain by andesite flows and breccias of the Weaver Lake Member. The WeaverLake Member consists of a >2600(+) m thick laterally and vertically variable succession of interdigitateddacite to andesite flows, flow breccias, tuffbreccia, lapilli tuff, tuff and epiclastic conglomerate, sandstone,and siltstone. Andesite to rhyolite dikes and sills are locally abundant, and a prominent rhyolite dome157i—.CD.-,.CDo—.QCD(00Cl)CDCDaOO C,I-tj:flNç,oCD•0!D.CDO2.0.C,CDCD.ot. P1•—-‘CDcomplex characterizes the upper part of the member on the east side of the outcrop belt (Fig. 6.2). Zirconrecovered from the rhyolite dome yields a U-Pb age of 166.0 +1- 0.4 Ma (Early Bathoman).The Weaver Lake Member is conformably overlain by volcaniclastic sedimentary rocks of the EchoIsland Member. The Echo Island Member comprises tuffaceous sandstone, siltstone, mudstone, granule tocobble conglomerate, and lesser tuffbreccia, lapilli tuff and crystal to vitric tuff. The member consists of thinto medium bedded, laterally continuous beds of volcanic sandstone, siltstone, mudstone, and crystal tuffintercalated with thick bedded to massive (2-15 m) tuffaceous sandstone, lapilli tuff, and tuffbreccia. Thecontact between the Echo Island Member and the overlying Mysterious Creek Formation is an angularunconformity.c. BiostratigraphyThe Harrison Lake Formation consists primarily of volcanic rocks and associated coarse-grainedclastic sediments, and is generally unfossiliferous. The Francis Lake Member, and, to a much lesser extent,the Weaver Lake Member, contain the only sparsely fossiliferous strata within the Harrison Lake Formation.Discovery of the ammonite Dactylloceras sp. (G.K. Jakobs, personal conununication, 1992) in argillaceousstrata intercalated with crystal tuff in the Francis Lake Member establishes the time of initiation of volcanismin the Harrison Lake Fonnation as late Early Toarcian. Age control in the upper part of the formation isprovided by U-Pb zircon age from the upper Weaver Lake Member (Chapter 4).Preservation of the fauna in dark grey argillite intercalated with probable partial turbidites argues fordeposition in a low-energy marine enviromnent below effective wave base. Pectimd bivalves and belemnitesare associated with ammonites in the Francis Lake Member (Arthur et al., 1993). Pectinid bivalves andbelemnites occur in the Weaver Lake Member (Pearson, 1973), and trigoniid bivalves and belemnites arereported from the Echo Island Member (Crickmay, 1925; Arthur et al., 1993). The occurrence of shallowwater benthic fauna throughout the formation suggests deposition in a nearshore shallow water environment.159In addition, aimnonite and bivalve fragments, together with wood debris, are commonly found at the base ofthick, graded sandstone beds, suggesting deposition by mass sediment gravity flows derived from nearshoreshallow water enviromnent.Boreal, Tethyan, and pandemic anunomte genera of Toarcian and Aalenian age have beendocumented in the Harrison Lake Formation (Table 6.1). The occurrence of this mixed fauna suggests theHarrison Lake Formation may have been deposited at mid-latitude in the Northern Hemisphere (Taylor et al.,1984).d. Volcanic GeochemistryPrimary volcanic rocks in the Harrison Lake Formation range from basaltic andesite to rhyolite, andare medium- to high-K calcaikaline in composition (Fig. 6.6 a,b). Trace element spidergrams display thestrongly spiked pattern indicative of subduction-related calcalkaline suites, and the rare earth element (REE)pattern is light rare earth element (LREE) enriched, which also supports a volcanic arc affinity for the rocks(Gill, 1981; Fig. 6.6c, d). Detailed analysis of the geochemistry of the Harrison Lake Formation is given inChapter 4, and comparison with arc volcanics of the Bonanza Group on Wrangellia is given in Chapter 5.e. Isotopic signatureNd and Sr isotopic analyses of primary volcanic rocks in the Harrison Lake Formation demonstratethe relatively juvemle nature of the arc system. Initial ENd values range from +3.53 to +6.97, and initial87Sr/6r range from 0.7026-0.7045. These values plot in the uncontaminated island arc volcanic (JAy) fieldon a ENd vs. 87Sr/6rdiagram (Fig. 6.7). However, incorporation of a minor amount of subducted sedimentor interaction with slightly evolved crust is indicated by more transitional ENd values (<+5; White andPatchett, 1984).160191(z861“es)snreADflupuotp02PSWUU0U‘Um1rnp1UtUtflJ(p‘(cs6i’uIu’PNpiwiop(ij)aiov02psijuuou‘wip2UWji1ppux(3(161)IT!OuioijwuiipjSAa(q‘(LL61)poiipiw1sq3uuzoij2ojdZoujiZSA;jq(uouuoWJ1I0SLU11tj3JO1U1N10JsUreJETpDTWtj30-99La(ppm)/T102gCIII0III11111111111III___\.:-0.:-\../8./—.---———\IIIIScimple/MORBSampIe/Chondte_____________EIIIIIIIULIsotopic values of flne-grained clastic sediments within the Harrison Lake Formation display a widerrange of initial ENd and 87Sr/6r than the primary volcanic rocks. Initial ENd values range from +1.98 to+5.21, and initial 87Sr/6rratios range from 0.7027-0.7056 (Fig. 6.7). Isotopic analyses of fine-grainedclastic sediments reflect the weighted average isotopic composition of the source region(s). Harrison LakeFormation sedimentary rocks have slightly lower ENd values than associated primary volcanic rocks,indicating that the sedimentary isotopic signature is a mixture ofjuvenile arc detritus and a more evolveddetrital component. The strontium isotopic ratios for the sediments display a significantly wider range thanthe majority of the volcanic rocks. Higher values of87Sr/6rcan be the result of hydrothermal alteration ofvolcanic rocks or incorporation of seawater strontium into the sediment (DePaolo and Wasserburg, 1977).Comparison of ENd values and stratigraphic position indicates a temporal control on isotopicfluctuations (Fig. 6.8). In the upper portion of the Celia Cove Member and lower portion of the Francis LakeMember, sediment ENd values are similar to those determined for the underlying Triassic Camp CoveFormation and for volcanic rocks higher in the section (Table 6.3). These values are consistent withderivation of the sediment from either the underlying strata or from laterally adjacent volcanic rocks of theBonanza-Harrison arc system (Chapter 5). A significant isotopic excursion is evident in Francis Lake Membersediments of Late Toarcian age, as indicated by a drop in ENd values of up to three epsilon units. These moreevolved values could not have been derived from subjacent strata or from laterally adjacent volcanic rocks, andrequire incorporation of more evolved sediment into the basin in Late Toarcian time. Isotopic values shiftback to more juvenile values in the Aalenian, coincident with the main effusive phase of Harrison Lakevolcanism, and remain juvenile until the earliest Bathonian. Slightly more evolved values near the top of thesection are related to the most silicic period of volcanism, and may reflect partial melting of slightly oldercrustal materials during the evolution of the magmatic system (DePaolo, 1988; Chapter 4).16210.00__________ _______________8 00 —• Hamson Lk Fm volcanic mcks600-• Harnson Lk Fm sedimentaiy mcb400_•1AVu 12.00___•+ • ‘I.6Nd 000 CHUR-200 IIAVc-400_-600_ ToPrecambrianreJI I“iii,. ,0.7020 0.7040 0.7060 0.7080 0.710087S/86SrFigure 6.7- ENd vs. 87Sr/86Sr isotopic diagram for Harrison Lake Formation. Note arc fields and position ofsamples relative to mantle array. Data from DePaolo (1988), Samson et al. (1991), Hawkesworth(1993). IAVc=contaminated island arc; IAVu= uncontaminated island arc; MORBmid ocean ridgebasalt; CHUR=chondritic values.163$ I222IIF’; Ii) m z ci••cicu22..f Depositional EnvironmentThe Harrison Lake Fonnation may be described as a classic island arc sequence (Chapter 4), and datapresented here reflects the evolution of the island arc sequence through time (Laure et aL, 1991). The CeliaCove Member is a basal conglomerate that indicates crustal instability and erosion of Triassic basement,probably due to thermal expansion of the crust induced by subduction of oceanic crust (Laure et al., 1991;Chapter 4). The conglomerate was deposited in a subaqueous environment by mass sediment gravity flows,and is overlain by partial turbidite and hemipelagic deposits. Low-energy hemipelagic deposition wasinterrupted in the early Late Toarcian by deposition of thick sequences of primary pyroclastic andresedimented pyroclastic sediments, marking the initiation of explosive volcanism in the Harrison LakeFormation.The transition from tuffaceous sedimentary rocks of the Francis Lake Member to lava flows andvolcanic breccias of the Weaver Lake Member indicates submarine deposition of resedimented pyroclasticdebris generated by explosive volcanism slowed in the Aalenian, and was superceded by effusive volcanism.Deposition of the Weaver Lake Member was characterised by the accumulation of thick sequences of andesiticto dacitic flows, tuff breccias, agglomerates, hyaloclastites, minor pyroclastic deposits, and epiclasticconglomerates, sandstone, and siltstone. Submarine volcanism is indicated by hyaloclastites and theoccurrence of shallow marine fauna, whereas adjacent subaerial deposition is suggested by the abundance ofwood debris and well rounded, channelized, epiclastic conglomerate. Composition of the volcanic rocks isapparently controlled by fractional ciystallization; flows evolve from basaltic andesite near the base to rhyoliticdikes and domes near the contact with the Echo Island Member (Chapter 4). The transition from flows andbreccias of the Weaver Lake Member to the well-stratified pyroclastic debris of the Echo Island Memberreflects the change from dominantly effusive volcanism to explosive volcanism in the upper part of theformation.165The Harrison Lake Formation is unconformably overlain by the Callovian Mysterious CreekFormation. Overturned bedding and mesoscale folds in the Echo Island Member indicates the contact is anangular unconformity, and suggests a Bathonian deformational event. The Mysterious Creek Formationcontains shale, siltstone, and sandstone deposited in shallow marine to below wavebase environments (Arthuretal., 1993).B. Cadwallader Terranea. Terrane DescriptionThe Cadwallader terrane consists of Tnassic volcanic and volcaniclastic rocks sequentially overlainby Upper Triassic, Lower to Middle Jurassic, Middle to Upper Jurassic and Lower Cretaceous elasticsuccessions (Rusmore, 1987; Umhoefer, 1989; Fig. 6.4). The base of the terrane contains volcanic arc rocksand conformably overlying turbidite strata of the Cadwallader Group (Carnian to Norian; Rusmore, 1987).The Cadwallader Group is interpreted to be conformably overlain by fluvial to shallow marine elastic rocksand limestones of the Tyaughton Group (middle to upper Norian; Umhoefer, 1990). The Last CreekFormation (Hettangian to Bajocian) comprises conglomerate, sandstone, siltstone and shale unconformablyabove the Tyaughton Group (O’Brien, 1985; Umhoefer, 1990). Marine sandstone, siltstone, and shale of theRelay Mountain Group (Callovian to Barremian) is interpreted to unconformably overlie the Last CreekFonnation, and these rocks are in unconformably overlain by coarse clastic rocks of the Taylor Creek Group(Barremian to Albian; Umhoefer, 1989; Garver, 1989, 1992).Middle Jurassic strata of the Last Creek Formation form a thin (<400 m), poorly exposed portion ofthe >5 km thick Cadwallader terrane. The formation has a limited areal extent, and is restricted to the LastCreek drainage and ridges west of Castle Pass (Plate 1). Correlative strata are sporadically exposed to thenorthwest, in the southwestern portion of the Taseko Lakes map area (Tipper, 1978) and in the southeasternportion of the Mt. Waddington map area to the northwest (Tipper, 1969; Umhoefer and Tipper, 1991;166Schiarizza et al., 1994). Despite limited exposure, the Last Creek Formation has received a great deal of studybecause of its remarkable anunonite fauna (Frebold, 1967; Frebold et al., 1969; Tipper, 1978; O’Brien, 1985;Poulton and Tipper, 1991; Jakobs, 1992). A detailed analysis of the Last Creek Formation and subjacent stratais the subject of an Geological Survey of Canada bulletin in preparation by H.W. Tipper and P.3. Umhoefer(H.W. Tipper, personal communication, 1994), and their nomenclature is used herein. The followinglithologic descriptions are based on reconnaissance stratigraphic studies by the author in 1991 and 1993,supplemented with data from Umhoefer (1989) and Umhoefer and Tipper (1991).b. LithostratigraphyThe Last Creek Fonnation consists of Upper Hettangian to Upper Bajocian clastic strata that havebeen subdivided into two members (Poulton, 1994; H.W. Tipper, personal communication, 1994; Fig. 6.9).The Castle Pass member comprises Upper Hettangian to Upper Sinemurian conglomerate, sandstone, siltstone,and shale disconformably overlying Upper Norian strata of the Tyaughton Group (Umhoefer, 1989). Thebasal conglomerate is a distinctive unit that consists of matrix-supported granule to pebble-size clasts ofvolcanics, chert, and sedimentaiy lithics set in a coarse lithic wacke matrix. The conglomerate has a bimodalgrain size distribution, consisting of granule-size angular chert lithic clasts (45%) and pebble to cobble,subrounded volcanic clasts (51%). Ammonite fragments and wood debris are common, and channels arelocally evident (O’Brien, 1985; Umhoefer, 1989). The conglomerate